drivers: mfd: wcd9xxx: Add support to configure dmic clock

DMIC clock on ECPP(Echo Cancellation Ping-Pong) path can be set to
lower clock rate to avoid power consumption. Change configures the
DMIC clock rate on ECPP path based on device tree settings.

CRs-fixed: 1022917
Change-Id: I03e1a7f5ef3cf1b1907c03a3d38965ce3a611bc4
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>
Signed-off-by: Vidyakumar Athota <vathota@codeaurora.org>