msm: pm: Prevent system sleeps when suspend is in progress

The MPM driver sends a XO vote for suspend during suspend prepare
callback. If the CPU were to execute a system low power mode after
suspend, the MPM driver votes against XO shutdown based on the list of
enabled interrupts. This vote is never removed and results in the XO
vote for shutdown overridden by the idle parameters.

Fix issue by ensuring that the system doesn't enter system low power
mode when the device is preparing for suspend. This would also prevent
RPM power collapse as RPM could turn off XO based on state of the
suspend votes sent as a part of suspend_prepare.

CRs-fixed: 551806
Change-Id: I9104f28898c995f69b8c0b9124651b915e15f690
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>