Add SPI pins for BeagleBone Black

Refer to header default pin modes
http://beagleboard.org/static/images/cape-headers.png

P9_17 (SPI0_CSO => CE0) enables peripheral device
P9_18 (SPI0_D1 => MOSI) outputs data to peripheral device
P9_21 (SPIO_DO => MISO) receives data from peripheral device
P9_22 (SPI0_SCLK => SCLK) outputs clock signal

Use config-pin to set pin mode for SPI pins
https://github.com/beagleboard/bb.org-overlays/tree/master/tools/beaglebone-universal-io

config-pin p9.17 spi_cs
config-pin p9.18 spi
config-pin p9.21 spi
config-pin p9.22 spi_sclk