SDEI: Mask events after CPU wakeup

The specification requires that, after wakeup from a CPU suspend, the
dispatcher must mask all events on the CPU. This patch adds the feature
to the SDEI dispatcher by subscribing to the PSCI suspend to power down
event, and masking all events on the PE.

Change-Id: I9fe1d1bc2a58379ba7bba953a8d8b275fc18902c
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>