OMAPDSS: HDMI: Add support to dump registers through debugfs

Add support to dump the HDMI wrapper, core, PLL and PHY registers
through debugfs.

Signed-off-by: Mythri P K <mythripk@ti.com>
[tomi.valkeinen@ti.com: updated the description]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>