fs/resctrl: Add support for assigning iommu_groups to resctrl groups

Arm's MPAM has support for assigning devices behind an IOMMU to a
control or monitor group. This can be used for device-passthrough
for a VM, or user-space drivers using VFIO to ensure the device
is either in the same control group as the CPU threads.
Alternatively, the iommu_group may be assigned to a different
control group with preferential schema values.

Extend the resctrl tasks file to include iommu_groups. These
appear as 'iommu_group:0', where 0 is the group number that
can be found from /sys/kernel/iommu_groups/. iommu_groups
can be moved between resctrl groups by writing this string
in the same way as tasks are moved.
No state is preserved by resctrl, an iommu_group that disappears
will no longer be listed as being part of a resctrl group. A new
iommu_group will appear in the default group.

Add helpers to list and move iommu_groups. Architecture specific
helpers are used to apply the closid/rmid to the iommu_group due
to the way MPAM emulates CDP.

Tested-by: Amit Singh Tomar <amitsinght@marvell.com>
Signed-off-by: James Morse <james.morse@arm.com>