cnss: Add support for external 1.15V supply for QCA6174

QCA6174 SWREG could be driven thru internal PMU or external 1.15V.
Add support to vote for the optional SWREG external supply.

CRs-Fixed: 655624
Change-Id: I22d8ca3e40b59c48cce527ca1de331a76abfcb39
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>