arm-cci: Refactor CCI PMU enable/disable methods

This patch refactors the CCI PMU driver code a little bit to
make it easier share the code for enabling/disabling the CCI
PMU. This will be used by the hooks to work around the special cases
where writing to a counter is not always that easy(e.g, CCI-500)

No functional changes.

Cc: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>