dt-bindings: sound: gtm601: add the broadmobi interface

The Broadmobi BM818 uses a different sample rate and channels from the
option modem.

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191223154712.18581-3-angus@akkea.ca


Signed-off-by: Mark Brown <broonie@kernel.org>