feat(rme): add ENABLE_RME build option and support for RMM image

The changes include:

- A new build option (ENABLE_RME) to enable FEAT_RME

- New image called RMM. RMM is R-EL2 firmware that manages Realms.
  When building TF-A, a path to RMM image can be specified using
  the "RMM" build flag. If RMM image is not provided, TRP is built
  by default and used as RMM image.

- Support for RMM image in fiptool

Signed-off-by: Zelalem Aweke <zelalem.aweke@arm.com>
Change-Id: I017c23ef02e465a5198baafd665a60858ecd1b25
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 7fe6ccd..1259881 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -271,6 +271,10 @@
    be enabled. If ``ENABLE_PMF`` is set, the residency statistics are tracked in
    software.
 
+- ``ENABLE_RME``: Boolean option to enable support for the ARMv9 Realm
+   Management Extension. Default value is 0. This is currently an experimental
+   feature.
+
 -  ``ENABLE_RUNTIME_INSTRUMENTATION``: Boolean option to enable runtime
    instrumentation which injects timestamp collection points into TF-A to
    allow runtime performance to be measured. Currently, only PSCI is