arm-cci500: Don't enable PMU driver by default

Disable building CCI-500 PMU driver by default.

Reported-by: Russell King <linux@arm.linux.org.uk>
Cc: Punit Agrawal <punit.agrawal@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: arm@kernel.org
Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>