lib: sbi: Fix mhpmeventh access for rv32 in absence of sscofpmf

MHPMEVENT3H-31H are defined in sscofpmf extension. Thus, they should be
accessed only if sscofpmf is present.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>