Add helper routines to check if the counter is enabled or not.

Cc: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: NOlof Johansson <olof@lixom.net>
Acked-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>