lib: sbi: Add PMU specific platform hooks

A platform hook to initialize PMU allows platform vendors to provide
their own mechanism to define pmu event-counter mappings in addition
to the DT based approach.

Another platform hook that allows platform vendors customize the
final mhpmevent value configuration.

Reviewed-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>