ArmPkg/MmCommunicationDxe: relay architected PI events to MM context

ArmPkg/MmCommunicationDxe: relay architected PI events to MM context

PI defines a few architected events that have significance in the MM
context as well as in the non-secure DXE context. So register notify
handlers for these events, and relay them into the standalone MM world.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Achin Gupta <achin.gupta@arm.com>