sbi: sbi_domain_context: Check privilege spec version before accessing S-mode CSRs

SCOUNTEREN and SENVCFG may not be supported on certain RISC-V core,
so check the existence of these CSRs via privilege spec version to
prevent illegal instructions.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Signed-off-by: Alvin Chang <alvinga@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>