fix(pmu): make MDCR_EL3.MTPME=1 out of reset

Make the default value for MTPME always be 1 to preserve the reset
behaviour on newer revisions and on older revisions where the bit is
RES0 it doesn't matter.

Before its introduction MDCR_EL3.MTPME was RES0. Upon its introduction
the field resets to 1, making the MTPMU architecturally "enabled". As
such, the logical action on TF-A's part is to "disable" it, which led to
the introduction of DISABLE_MTPMU.

This hinges on the assumption that MDCR_EL3.MTPME will always be 1
unless the above flag is set. Unfortunately this is not the case, as the
reset value is overwritten at reset with a macro that sets this bit to
0.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ie570774972f246b3aa41dc016ecbcc6fc2f581f6