lib: sbi_pmu: Add custom PMU device operations

We extend SBI PMU implementation to allow custom PMU device operations
which a platform can use for platform specific quirks.

The custom PMU device operations added by this patch include:
1) Operations to allow a platform implement custom firmware events.
   These custom firmware events can be SBI vendor extension related
   events or platform specific per-HART events are not possible to
   count through HPM CSRs.
2) Operations to allow a platform implement custom way for enabling
   (or disabling) an overflow interrupt (e.g. T-Head C9xx).

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>