dt-bindings: display: Add compatible for A64 HDMI

The HDMI controller on Allwinner A64 is similar on the one on
H3/H5/A83T (although the PHY is different with A83T).

Add A64 compatible and append A83T compatible as fallback.
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Rob Herring <robh@kernel.org>
[Icenowy: refactor commit log]
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904044053.15425-7-icenowy@aosc.io

Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt

@@ -78,6 +78,7 @@ Required properties:
 
   - compatible: value must be one of:
     * "allwinner,sun8i-a83t-dw-hdmi"
+    * "allwinner,sun50i-a64-dw-hdmi", "allwinner,sun8i-a83t-dw-hdmi"
   - reg: base address and size of memory-mapped region
   - reg-io-width: See dw_hdmi.txt. Shall be 1.
   - interrupts: HDMI interrupt number