arm-cci: simplify sysfs attr handling

There's no need to dynamically initialise attribute pointers when we can
get the compiler to do it for us. We also don't need a dev_ext_attribute
for the cpumask, as the drvdata for a PMU device is a pointer to struct
pmu.

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