fix(mpam): refine MPAM initialization and enablement process

Restricts MPAM to only NS world and enables trap to EL3 for access of
MPAM registers from lower ELs of Secure and Realm world.

This patch removes MPAM enablement from global context and adds it to
EL3 State context which enables/disables MPAM during world switches.
Renamed ENABLE_MPAM_FOR_LOWER_ELS to ENABLE_FEAT_MPAM and
removed mpam_init_el3() as RESET behaviour is trapping.

Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>
Change-Id: I131f9dba5df236a71959b2d425ee11af7f3c38c4

docs/getting_started/build-options.rst

@@ -397,7 +397,7 @@ Common build options
    support in GCC for TF-A. This option is currently only supported for
    AArch64. Default is 0.
 
--  ``ENABLE_MPAM_FOR_LOWER_ELS``: Numeric value to enable lower ELs to use MPAM
+-  ``ENABLE_FEAT_MPAM``: Numeric value to enable lower ELs to use MPAM
    feature. MPAM is an optional Armv8.4 extension that enables various memory
    system components and resources to define partitions; software running at
    various ELs can assign themselves to desired partition to control their
@@ -408,7 +408,9 @@ Common build options
    access their own MPAM registers without trapping into EL3. This option
    doesn't make use of partitioning in EL3, however. Platform initialisation
    code should configure and use partitions in EL3 as required. This option
-   defaults to ``0``.
+   defaults to ``2`` since MPAM is enabled by default for NS world only.
+   The flag is automatically disabled when the target
+   architecture is AArch32.
 
 -  ``ENABLE_MPMM``: Boolean option to enable support for the Maximum Power
    Mitigation Mechanism supported by certain Arm cores, which allows the SoC