SDEI: Unconditionally resume Secure if it was interrupted

Secure world execution nearly always expect a controlled exit to
Non-secure world. SDEI interrupts, although targets EL3, occur on behalf
of Non-secure world, and may have higher priority than Secure world
interrupts. Therefore they might preempt Secure execution, and yield
execution to Non-secure SDEI handler. Upon completion of SDEI event
handling (regardless of whether it's COPLETE or COMPLETE_AND_RESUME), we
must resume Secure execution if it was preempted.

Change-Id: I6edd991032588588427ba2fe6c3d7668f7080e3d
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>