lib: sbi: Update csr_read/write_num for PMU

The Sscofpmf extension introduces mhpmevent[h] csrs to handle filtering
/overflow bits in RV32. There is no way to read/write mcountinhibit
using mcountinhibit csr using a variable.

Updated the support to read/write mhpmevent[h] and mcountinhibit csr.

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>