drm/bridge/synopsys: dw-hdmi: Export some PHY related functions

Parts of PHY code could be useful also for custom PHYs. For example,
Allwinner A83T has custom PHY which is probably Synopsys gen2 PHY
with few additional memory mapped registers, so most of the Synopsys PHY
related code could be reused.

Functions exported here are actually not specific to Synopsys PHYs but
to DWC HDMI controller PHY interface. This means that even if the PHY is
completely custom, i.e. not designed by Synopsys, exported functions can
be useful.

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
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/20180214200906.31509-5-jernej.skrabec@siol.net