arm_pmu: fold platform helpers into platform code

The armpmu_{request,free}_irqs() helpers are only used by
arm_pmu_platform.c, so let's fold them in and make them static.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>