msm: spm: Store status register when programming low power mode

The SPM status register contains information about the last low power mode
that was executed by the SPM. Storing the status register when programming
an SPM provides information on what the last low power mode was executed by
the SPM and is useful in debugging crashes associated with warmboot.

Change-Id: I69a9d1edb781de5999e71ddc75e8265bfa382742
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>