feat(mpmm): add support for MPMM

MPMM - the Maximum Power Mitigation Mechanism - is an optional
microarchitectural feature present on some Armv9-A cores, introduced
with the Cortex-X2, Cortex-A710 and Cortex-A510 cores.

MPMM allows the SoC firmware to detect and limit high activity events
to assist in SoC processor power domain dynamic power budgeting and
limit the triggering of whole-rail (i.e. clock chopping) responses to
overcurrent conditions.

This feature is enabled via the `ENABLE_MPMM` build option.
Configuration can be done via FCONF by enabling `ENABLE_MPMM_FCONF`, or
by via the plaform-implemented `plat_mpmm_topology` function.

Change-Id: I77da82808ad4744ece8263f0bf215c5a091c3167
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/docs/components/index.rst b/docs/components/index.rst
index e467516..754526d 100644
--- a/docs/components/index.rst
+++ b/docs/components/index.rst
@@ -14,6 +14,7 @@
    fconf/index
    firmware-update
    measured_boot/index
+   mpmm
    platform-interrupt-controller-API
    ras
    romlib-design