drivers: CCI: fix used_mask init in validate_group()

Currently in validate_group(), there is a static initializer
for fake_pmu.used_mask which is based on CPU_BITS_NONE but
the used_mask array size is based on CCI_PMU_MAX_HW_EVENTS.
CCI_PMU_MAX_HW_EVENTS is not based on NR_CPUS, so CPU_BITS_NONE
is not correct and will cause a build failure if NR_CPUS
is set high enough to make CPU_BITS_NONE larger than used_mask.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>