arm_mpam: resctrl: Re-synchronise resctrl's view of online CPUs

So far we probe devices as they become accessible. Each time we probe
a new device, we eagerly schedule mpam_enable(). Once all the
devices have been probed mpam_enable() will poke resctrl.

At this point, resctrl has an inconcistent view of which CPUs are online,
as we only update the the classes that we picked, and we only
did that after there were enough CPUs online to have probed all
the devices.

Instead of having some complicated re-sync logic, unregister
the cpuhp callbacks, register resctrl, then re-register them.
As we know all the devices have been probed, no-one can find a new
one to cause mpam_enable() to be re-scheduled.

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