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