regulator: rpm-smd: Add snapshot of rpm-smd regulator driver

Add snapshot of rpm-smd regulator driver from msm-4.19.
This snapshot is taken as of msm-4.19 commit 15bff51c0329
("smcinvoke : Add locking to shared variables").

Change-Id: I96cd163e490f811c0564dbdf30fb250156cffc85
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>