drm/sun4i: DW HDMI PHY: Add support for second PLL

Some DW HDMI PHYs, like those found in A64 and R40 SoCs, can select
between two clock parents.

Add code which reads second PLL from DT.

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/20180625120304.7543-19-jernej.skrabec@siol.net