arm_mpam: resctrl: Add iommu helpers to get/set the partid and pmg

SMMU that support MPAM can be configured to use a particular partid
and pmg for a stream. The assignment of an iommu_group and its
corresponding streams should be done via resctrl.

Add helpers similar to setting a closid/rmid on a task. We need
the same shifting if the CPUs are using CDP. The SMMU only takes
one partid, conceptually its always making data accesses.

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