regulator: mem-acc-regulator: Add support for updating mem type registers

Some targets need to change mem-acc-type registers for mode switching. Add
optional device tree properties and driver support for the need.

CRs-Fixed: 780594
CRs-Fixed: 780605
Change-Id: I14913d7fbfb73bd88f675752b30bf6f469380656
Signed-off-by: Ke Liu <keliu@codeaurora.org>