drivers: lmh-dcvsh: Add isens vref regulator support

Isens hardware needs the vref LDO to be on for its working. Inorder to
enable this, LMH DCVSh driver will place a APSS active only vote for the
vref.

Add support in the LMH DCVSh driver to probe for the isens active only vref
regulator and enable it during probe.

Change-Id: I8d55efc21d53ca440cef9fe0a7256a2fe753bdd0
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>