lib: sbi: Delegate PMU counter overflow interrupt to S mode

OpenSBI doesn't handle PMU counters for now.

Delegate the overflow counter to S-mode always.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>