mfd: sun6i-prcm: Add codec analog controls sub-device for Allwinner A23

The PRCM block on the A23 contains a message box like interface to
the registers for the analog path controls of the internal codec.

Add a sub-device for it.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>