ArmPkg/MmCommunicationDxe: fix logic bug in DXE driver entrypoint

ArmPkg/MmCommunicationDxe: fix logic bug in DXE driver entrypoint

Commit 2fe25a74d6fee3c2 ("ArmPkg/MmCommunicationDxe: relay architected PI
events to MM context") update the ARM specific standalone MM client
driver to register for certain events in the entrypoint code, but did
so in a way that makes the entrypoint always return with an error.

Instead, return EFI_SUCCESS if registering for those events succeeds,
and back out the registrations that did succeed if one fails, and
return an error.

Fixes: 2fe25a74d6fee3c2 ("ArmPkg/MmCommunicationDxe: relay architected PI events to MM context")
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>