MPAM: enable MPAM EL2 traps

Complete the MPAM enablement in TF-A for lower ELs by enabling the EL2
traps in MPAMHCR_EL2 and MPAM2_EL2.This prevents an
MPAM-unaware-hypervisor to be restricted by an MPAM-aware-guest.

Change-Id: I47bf3f833fa22baa590f83d49cc0e3f2974e698d
Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>