drivers/perf: riscv: Export PMU event info function

drivers/perf: riscv: Export PMU event info function

The event mapping function can be used in event info function to find out
the corresponding SBI PMU event encoding during the get_event_info function
as well. Refactor and export it so that it can be invoked from kvm and
internal driver.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Acked-by: Paul Walmsley <pjw@kernel.org>
Link: https://lore.kernel.org/r/20250909-pmu_event_info-v6-5-d8f80cacb884@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>