checkpatch: Add check for vreg_xxx api in opensource drivers

Vreg API implementation is deprecated. We are using
Linux regulator framework now. Hence vreg API should
not be used.

Change-Id: I8e31dac66592d2d195d190b770a436e93206cf8b
Signed-off-by: Pankaj Kumar <pakuma@codeaurora.org>
(cherry picked from commit adfb933a24911e3514a2f1b6fe1b1a9151fec56d)
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>