dt-bindings: display: sun4i-drm: Add description of A64 HDMI PHY

A64 HDMI PHY is similar to H3 HDMI PHY except it has two possible PLL
clock parents. It is compatible to other HDMI PHYs, like that found in
R40.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180625120304.7543-16-jernej.skrabec@siol.net

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

@@ -101,6 +101,7 @@ DWC HDMI PHY
 
 Required properties:
   - compatible: value must be one of:
+    * allwinner,sun50i-a64-hdmi-phy
     * allwinner,sun8i-a83t-hdmi-phy
     * allwinner,sun8i-h3-hdmi-phy
   - reg: base address and size of memory-mapped region
@@ -111,8 +112,9 @@ Required properties:
   - resets: phandle to the reset controller driving the PHY
   - reset-names: must be "phy"
 
-H3 HDMI PHY requires additional clock:
+H3 and A64 HDMI PHY require additional clocks:
   - pll-0: parent of phy clock
+  - pll-1: second possible phy clock parent (A64 only)
 
 TV Encoder
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