arm_mpam: Rename a misleading resctrl hook parameter

The resctrl_arch_set_cpu_default_closid_rmid() arch resctrl hook
has a parameter that is used by the resctrl core code to pass in
the generic RMID value.  However, this parameter is currently named
"pmg" in the signature arm64's version of this function.

Currently the PMG and RMID are always the same, but they may become
different under PARTID/PMG remapping transformations.

Fix the name to "rmid", to match what resctrl is actually passing.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>