Add support for Allwinner H3 audio codec.

The audio controller in the H3 is more or less the same as A10/A20 except
some registers are shuffled around. The mixer interface, however, is
completely different between SoCs. Separate a10_mixer_class and
h3_mixer_class implementations are now made available. This will also make
adding support for other SoCs easier in the future.

Reviewed by: andrew, ganbold
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D8425