drivers: CCI: add ARM CCI PMU support

Extend the existing CCI driver to support the PMU by registering a perf
backend for it.

Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Nicolas Pitre <nico@linaro.org>
Cc: Dave Martin <dave.martin@linaro.org>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
[will: removed broken __init annotations]
Signed-off-by: Will Deacon <will.deacon@arm.com>