dt-bindings: display: rockchip: dw-hdmi: Add regulator support

The RK3568 has HDMI_TX_AVDD0V9 and HDMI_TX_AVDD_1V8 supply inputs
needed for the HDMI port. Add the binding for these supplies.

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/20220422072841.2206452-11-s.hauer@pengutronix.de