lib: sbi: Allow programmable counters to monitor cycle/instret events

A platform may use programmable counters for cycle/instret events.
The priv spec allows that provided that cycle/instret also report those
events in addition to the programmable counters. We should allow that
functionality in OpenSBI.

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>