msm: spm: Add new API to set only SPM start address

The start addresses for different L2 low power modes could
be different between HLOSes. So HLOS cannot rely on secure
world to correctly program the L2 SPM.

Program L2 spm start address in HLOS and allow secure world
to configure only spm enable bit.

Change-Id: I3ae5d7a1574808a7fc33ab7d62294592fdc1fdf8
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>