ASoC: msm8974: Add support for external Class-D PAs

External Class-D PAs are LiQUID8974 specific,
and are only used if explicitly defined in DT.
Enable signal comes from PMIC8841 MPP01.

Change-Id: I8ea0de11b64eeeebcc4db3c29e143cae3e0cc16f
Signed-off-by: Eugene Yasman <eyasman@codeaurora.org>
Signed-off-by: Tanya Finkel <tfinkel@codeaurora.org>