M2: regulators: fixup: make RPM_VREG_ID_PM8921_S2 regulator sleep_selectable

M2: ATT: Increase frequency for RPM_VREG_ID_PM8921_S2 regulator to 2.4

(same as in stock. ATT specific change is forced in rpm-regulator.c)

Change-Id: Ib351f8f75437e85fb50ba166dcaf887fac042df5