drivers: cci: Export CCI PMU revision

The event numbering changed between revision r0 and r1 of the CCI
PMU. Expose this to userspace to allow tooling to handle the
differences in event numbers.

Suggested-by: Drew Richardson <Drew.Richardson@arm.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>