perf: xgene: Move PMU leaf functions into function pointer structure

This patch moves PMU leaf functions into a function pointer structure.
It helps code maintain and expasion easier.

Signed-off-by: Hoan Tran <hotran@apm.com>
[Mark: remove redundant cast]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
[will: make xgene_pmu_ops static]
Signed-off-by: Will Deacon <will.deacon@arm.com>