spi: shell: Add command to include CS GPIO in SPI device

This allows to configure a GPIO pin as a CS line, in order to perform
spi transfers to a particular device in the bus.

Example usage for using GPIO1.12 with GPIO_ACTIVE_LOW:
`spi cs gpio1 12 0x01`

Signed-off-by: Luis Ubieda <luisf@croxel.com>