ASoC: wcd934x: add low power mode support for codec

Codec goes to digital power collapse state and disables the external
clock when there is no audio usecase active. It is possible certain
low power audio usecases need the codec to be out of digital core
power collapse but can run using the internal codec clock. Change
adds support to turn on codec digital core without turning on the
external clock if the clock mode is set to internal.

Change-Id: I74c296d9b967ea56c3f03514022b259856533d9b
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>