MAINTAINERS: update references to stm32 audio bindings

Changeset 81437cc3

 ("Merge series "dt-bindings: stm32: convert audio dfsdm to json-schema" from Olivier Moysan <olivier.moysan@st.com>:")
removed bindings/sound/st,stm32-adfsdm.txt, as stm32-* audio
bindings are now under: bindings/iio/adc/st,stm32-*.yaml.

Update cross-references to them accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/03950bbd5cf7bac10eaaff3725e283d3ec2538c5.1610536535.git.mchehab+huawei@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>