msm: mdss: Support additional vreg fields in dtsi

Add support for additonal vreg fields like
sleeps for before and after vreg enable/disable.
These changes are to support updated dtsi
with these additional fields.

Change-Id: Ib8cd9d870965ba1d20406d694e2c6ca6810b38d3
Signed-off-by: Arpita Banerjee <cabane@codeaurora.org>