arm_mpam: Allow MBWU counters to be used, even when not free running

Resctrl exposes the cache opccupancy and bandwidth counters to user space
via files. Each control and monitor group has a copy of these files, and
can read the counter by reading the file.

MPAM needs to allocate a hardware monitor to do the counting work, and there
may not be enough for every control and monitor group to have one. Currently
MPAM doesn't expose the MBM/MBWU counters to resctrl unless there are enough
monitors.

To allow perf to read these counters via a PMU driver, add support for
allocating a monitor. To prevent these appearing in the resctrl filesystem,
report false from resctrl_arch_event_is_free_running() for the MBM event
types.

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