[bsp][synopsys][emsk] add the doc of emsk

add the documenatation

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>

bsp/synopsys/emsk_em9d/README.md

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+# Synopsys DesignWare ARC EM Starter Kit
+
+## Overview
+
+The DesignWare™ ARC™ EM Starter Kit(EMSK, emsk) is a low-cost, versatile
+solution enabling rapid software development and software debugging, and
+profiling for the ARC EM Family of processors. The EM Family includes the EM4,
+EM6, EM5D, EM7D, EM9D, and EM11D cores.
+
+![EM Starter Kit](figures/emsk_board.jpg)
+
+The EMSK consists of a hardware platform and a software package, including
+pre-installed FPGA images of different configurations of the ARC EM Processor
+with peripherals.
+
+The development board is based on a Xilinx Spartan-6 LX45 FPGA. It supports
+hardware extensions using six 2x6 connectors supporting a total of 48 user I/O
+pins (plus power and ground pins) that can be used to connect components such
+as sensors, actuators, memories, displays, buttons, switches, and
+communication devices. A Digilent Pmod™ compatible extension board containing
+a four-channel 12-bit A/D converter with an IIC interface and an AC power
+adapter are included in the package.
+
+## Board Resources
+
+| Hardware | Description |
+| -- | -- |
+| SOC | EMSK |
+| Core | EM9D                                   |
+| Arch       |  32-bit ARC EM                                         |
+| Frequency  |  20/25 Mhz                                            |
+| RAM | 128 MB DDR RAM + 256 KB Instruction CCM + 256 KB Data CCM |
+|Flash | 16 MB SPI Flash for FPGA configuration with 128 KB reserved for application |
+
+## Usage
+
+### FPGA Configuration
+
+Current, only em9d configuration is supported and tested for RT-Thread. However,
+it's can be easily applied to other configurations because all emsk configurations almost share the same memory map and periphreals.
+
+Please refer [EMSK configuration](https://embarc.org/embarc_osp/doc/build/html/board/emsk.html) for details.
+
+### Toolchain
+
+The ARC GNU Toolchain offers all of the benefits of open source tools, including complete source code and a large install base. The ARC GNU IDE Installer consists of Eclipse IDE with [ARC GNU plugin for Eclipse](https://github.com/foss-for-synopsys-dwc-arc-processors/arc_gnu_eclipse/releases), [ARC GNU prebuilt toolchain](https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/releases) and [OpenOCD for ARC](https://github.com/foss-for-synopsys-dwc-arc-processors/openocd>)
+
+Here, the ARC GNU toolchain is installed to `c:\arc\gnu`. If not, please change the path configuration in rtconfig.py.
+
+### Compile
+
+please run the following cmds to compile
+
+    cd <rt-thread-root>/bsp/synopsys/emsk_em9d
+    scons
+
+## Debug
+
+You need to  install [Zadig](http://zadig.akeo.ie) to replace the default FTDI driver with WinUSB driver. See [How to Use OpenOCD on Windows](https://github.com/foss-for-synopsys-dwc-arc-processors/arc_gnu_eclipse/wiki/How-to-Use-OpenOCD-on-Windows>) for more information.
+
+After compile, please use the following cmds to debug
+
+    scons  --gdb
+
+
+## Supported Drivers
+
+| Driver | Supported | Comment  |
+| ------ | ----  | :------:  |
+| UART1 |  yes |  USB-UART for shell & log |
+| UART0 | yes | PMOD A UART for UART modules, e.g. esp8266 |
+
+## Maintainer
+- [vonhust](https://github.com/vonhust)
+
+## Notes
+