drm/sun4i: hdmi: Implement I2C adapter for A10s DDC bus

The documentation for drm_do_get_edid in drivers/gpu/drm/drm_edid.c states:
"As in the general case the DDC bus is accessible by the kernel at the I2C
level, drivers must make all reasonable efforts to expose it as an I2C
adapter and use drm_get_edid() instead of abusing this function."

Exposing the DDC bus as an I2C adapter is more beneficial as it can be used
for purposes other than reading the EDID such as modifying the EDID or
using the HDMI DDC pins as an I2C bus through the I2C dev interface from
userspace (e.g. i2c-tools).

Implement this for A10s.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>