arm: perf/kvm: Use GENMASK for ARMV8_PMU_PMCR_N

This is so that FIELD_GET and FIELD_PREP can be used and that the fields
are in a consistent format to arm64/tools/sysreg

Signed-off-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20231211161331.1277825-3-james.clark@arm.com


Signed-off-by: Will Deacon <will@kernel.org>