SPM: Add critical section for IPC "wait" in backend

Add critical section for IPC "wait" ops in the backend,
since the thread state and asserted signal value of partition
may be changed by interrupt.

Also rename the arg name 'signals' to 'signal_mask' since it is
a set of signals to wait for.

Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>
Change-Id: Ia12607fad1238d8ea443461fd220d949166f4638
4 files changed