[media] Documentation: bindings: add documentation for ir-spi device driver

Document the ir-spi driver's binding which is a IR led driven
through the SPI line.

Signed-off-by: Andi Shyti <andi.shyti@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>