mfd: wcd9xxx: Make DMIC clock drive strength platform dependent

Digital microphone clock drive strength setting in the codec could
be platform dependent based on the type of microphone, etc. Add support
to make the DMIC clock drive strength configurable through device tree.

CRs-fixed: 938006
Change-Id: I8b17d985052098b56d012081a77b6dfc94553eca
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>