drm/sun4i: Add support for variants to DW HDMI PHY

There are multiple variants of DW HDMI PHYs in Allwinner SoCs. While
some things like clock and reset setup are the same, PHY configuration
differs a lot.

Split out code which is PHY specific to separate functions and create
a structure which holds pointers to those functions.

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/20180301213442.16677-11-jernej.skrabec@siol.net