drm/sun4i: hdmi: create a regmap for later use

The HDMI driver is written with readl/writel I/O to the registers.
However, to support the A31 variant, which has a different layout
for the DDC registers, it was recommended to use regfields to have
a cleaner implementation. To use regfields, we need to create an
underlying regmap.

This patch only adds the regmap. It does not convert the existing
driver accesses to use regmap.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010032008.682-5-wens@csie.org