media: dt: bindings: Update binding documentation for sunxi IR controller

This patch updates documentation for Device-Tree bindings for sunxi IR
controller and adds the new optional property for the base clock
frequency.

Signed-off-by: Philipp Rossak <embed3d@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>