dts: spi: Add optional cs-gpios to the spi bus controller binding
dts: spi: Add optional cs-gpios to the spi bus controller binding

Now that we can generate gpio info from dts lets add optional support
fot the cs-gpios property in the bus controller node.  This matches the
binding spec from Linux on how cs-gpios are handled.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

ldts/zephyr,finikorg/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,mbolivar/zephyr,kraj/zephyr,kraj/zephyr,finikorg/zephyr,punitvara/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,ldts/zephyr,nashif/zephyr,mbolivar/zephyr,punitvara/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,nashif/zephyr,finikorg/zephyr,kraj/zephyr,mbolivar/zephyr,ldts/zephyr,explora26/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,zephyriot/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,explora26/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyriot/zephyr,mbolivar/zephyr,finikorg/zephyr,Vudentz/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr