ASoC: sun4i-codec: Revise comments for register definition macros

This revises existing comments in the register definition macros
section, and adds a few more, so that readers can clearly identify
the types of control registers.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>