On GICv3 systems, as a side effect of adding provision to handle EL3
interrupts (unconditionally routing FIQs to EL3), pending Non-secure
interrupts (signalled as FIQs) may preempt execution in lower Secure ELs
[1]. This will inadvertently disrupt the semantics of Fast SMC
(previously called Atomic SMC) calls.

To retain semantics of Fast SMCs, the GIC PMR must be programmed to
prevent Non-secure interrupts from preempting Secure execution. To that
effect, two new functions in the Exception Handling Framework subscribe
to events introduced in an earlier commit:

  - Upon 'cm_exited_normal_world', the Non-secure PMR is stashed, and
    the PMR is programmed to the highest Non-secure interrupt priority.

  - Upon 'cm_entering_normal_world', the previously stashed Non-secure
    PMR is restored.

The above sequence however prevents Yielding SMCs from being preempted
by Non-secure interrupts as intended. To facilitate this, the public API
exc_allow_ns_preemption() ...