media: dt-bindings: gpio-ir-tx: add support for GPIO IR Transmitter

Document the device tree bindings for the GPIO IR Bit Banging
Transmitter.

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