dt-bindings: display: rockchip: document VOP gamma LUT address

Add the register specifier description for an
optional gamma LUT address.

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010194351.17940-2-ezequiel@collabora.com