* [bsp][nxp] add missing drv_spi.h
Otherwise when building spi users such as u8g2, gcc will complain can't
find the drv_spi.h.
* [bsp][nxp] enable spi6
* [bsp][nxp] add spi sample code which uses spi6 to loopback
Connect spi6's MISO <--> MSIO, I.E P3_20 and P3_22 with Dupont Line
The sample usage and output would be:
msh >spi_sample
spi rbuf : 0 1 2 3 4 5 6 7 8 9 a b c d e f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
spi loopback mode test over!
