KVM: riscv: selftests: Add SBI PMU extension definitions

The SBI PMU extension definition is required for upcoming SBI PMU
selftests.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20240420151741.962500-21-atishp@rivosinc.com


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