configs/rdn2cfg1: add ENABLE_MPAM_FOR_LOWER_ELS flag

For software at lower exception levels than EL3 to access MPAM
system registers without trapping, MPAM system register for EL3
should be configured accordingly. Define 'ENABLE_MPAM_FOR_LOWER_ELS'
build option provided by Arm Trusted Firmware for RD-N2-Cfg1 and set it
to '1' to enable MPAM access from lower ELs.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: I6c347cb18b0eecc40d78f57b28a20693f2000f67