msm: rpm-smd: call SMD API to mask/unmask SMD interrupt.

When the system goes to sleep, the power management code sends a sleep
set to RPM. The code executes with interrupts disabled so the power
management code polls for ack message from RPM. Since the SMD interrupt
is in a triggered state and irq handler hasn't had a chance to run, the
pending interrupt prevent the Core from entering a low power mode.

To prevent interrupt from waking the system up, disable/enable the RPM to
SMD interrupt while entering/exiting sleep mode, by calling into the SMD
API.

Change-Id: I38faa18e6b1d7f4cc92d72c3da4ac8ca29275444
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>