lib: sbi: Reset the mhpmevent value upon counter reset

The hardware solely relies on the event selector value in mhpmevent
to figure out what event to monitor using that counter. It should be
reset when counter reset happens.

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