infra/common: add documentation for MPAM-resctrl

MPAM-resctrl is a feature by which Arm's MPAM memory system components
(MSCs) could configured by the existing resctrl framework. resctrl
provides a filesystem view of the MPAM MSCs present in the system.
Generic filesystem commands can be used to query capabilities of all
the MSCs, as well as modify resource control features of the MSCs that
could be changed.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: I121f4a036d013c1713f3bc2d4622d78133459904