dt-bindings: sun4i-drm: add compatible for R40 HDMI PHY

The Allwinner R40 HDMI PHY is currently the only one that seems to be
able to select between two PLL inputs.

Add a compatible string for it, and the pll-1 clock input definition.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180916043409.62374-3-icenowy@aosc.io