arm_mpam: Set INTERNAL as needed when setting MSC controls

Currently, when an MSC implements PARTID narrowing, MPAMCFG_PART_SEL is
left set to the reqPARTID while programming resource controls in
an MSC.

The MPAM architecture does not guarantee that any particular resource
controls will be updated correctly in this scenario.  Instead,
MPAMCFG_PART_SEL must be written with the corresponding intPARTID and
with the INTERNAL bit set before attempting to program resource controls.
Only the PARTID->intPARTID mappings can be written without the INTERNAL
bit set in MPAMCFG_PART_SEL.

Fix it, by rewriting MPAMCFG_PART_SEL appropriately after setting the
intPARTID mapping.

The MPAMCFG_INTPARTID_INTERNAL flag is not currently accepted as
input to the __mpam_part_sel() helper.  In the interest of keeping
the code clean, break this helper up.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>