MAINTAINERS: add ASoC maintainers for sound dt-bindings

Sound dt-bindings are applied by ASoC maintainers and should be
submit to ASoC list in addition to the devicetree list.

Hence, add this information into the MAINTAINERS file.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>