msm: pil-q6v5-mss: Make MSS regulator voting optional

On some chipsets, the modem is able to vote for its
own regulator and proxy voting takes care of the vote
during modem bootup. Make the regulator voting optional
in these cases.

Change-Id: I7a3dfc7dc41c823d574967981735d3bd80dd9410
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>