lib: sbi: Fix mstatus_init() for RV32 when Sscofpmf is not available

The mhpmevent3h to mhpmevent31h CSRs are available on RV32 only when
Sscofpmf extension is available so mstatus_init() should set this
CSRs only when Sscofpmf extension is available.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>