arm64: RME: Initialize PMCR.N with number counter supported by RMM

Provide an accurate number of available PMU counters to userspace when
setting up a Realm.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>