dt-bindings: display: rockchip: dw-hdmi: Make unwedge pinctrl optional

None of the upstream device tree files has a "unwedge" pinctrl
specified. Make it optional.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Tested-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220422072841.2206452-16-s.hauer@pengutronix.de