dt-bindings: display: vop2: Add missing rockchip,grf property for rk3566/8

The clock polarity of RGB signal output is controlled by GRF, this
property is already being used in the current device tree, but
forgot to describe it as a required property in the binding file.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20250303034436.192400-6-andyshrk@163.com