lib: sbi: Always enable access for all counters

OpenSBI doesn't use any counters for its own usage. Thus, all the counters
can be made accessible for lower privilege mode always. However, the
mcountinhibit must be set so that the counter doesn't increment.
As a result, we don't have to enable/disable mcounteren at every start/stop.

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>