media: rc: meson-ir-tx: document device tree bindings

This patch adds binding documentation for the IR transmitter
available in Amlogic Meson SoCs.

Signed-off-by: Viktor Prutyanov <viktor.prutyanov@phystech.edu>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>