lib: sbi: Fix sbi_pmu_exit() for systems not having MCOUNTINHIBIT csr

The sbi_pmu_exit() crashes on systems not having MCOUNTINHIBIT csr
so to fix this we check SBI_HART_HAS_MCOUNTINHIBIT feature in
sbi_pmu_exit() and do nothing if it is not available.

Fixes: 13d40f21 ("lib: sbi: Add PMU support")
Signed-off-by: Anup Patel <anup.patel@wdc.com>