arm/dt: msm9625: Add a sleep-set-only PM8019 L12 regulator

Add a sleep-set-only regulator device node for PMIC 8019
regulator LDO 12 (VDD_MX).  Specify an initial voltage for this
regulator in order to facilitate VDD_MX minimization during RPM
assisted power collapse.

Also change the minimum voltage allowed for LDO 12 from 0.75 V
to 0.675 V in order to match documentation for the VDD_MX
retention level.

Change-Id: I8c1fce5c3c4a4cb44616383f281e0606493d2ece
Signed-off-by: David Collins <collinsd@codeaurora.org>