ANDROID: mpam: Add module-based task to partid mapping interface

Create a mountpoint at /sys/fs/mpam and present a VFS to examine the
PID:PARTID mapping. This is only a read-only interface, the writable part
comes in a subsequent commit.

Change-Id: I49bd49082e2e9b38598ad9a6abae4fb1f2559281
Signed-off-by: Valentin Schneider <Valentin.Schneider@arm.com>