KVM: riscv: selftests: Add SBI PMU selftest

This test implements basic sanity test and cycle/instret event
counting tests.

Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20240420151741.962500-22-atishp@rivosinc.com


Signed-off-by: Anup Patel <anup@brainfault.org>