arm/dt: msm9625: Enable low voltage scaling

Add a new lpm level that will allow scaling of system rails to lower
voltages in certain use cases. These could be cases where we want
system clock to be on but lower the voltage rails.

Change-Id: Ife18d466628e45d225a6358d7906146f3d9270db
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>