lib: sbi_pmu: Add PMU snapshot definitions

OpenSBI doesn't support SBI PMU snapshot yet as there is not much benefit
unless the multiple counters overflow at the same time.

Just add the definition and return not supported error at this moment. The
default returned error is also not supported. Thus, no functional change
intended.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Atish Patra <atishp@rivosinc.com>