RISC-V: KVM: Don't add SBI multi regs in get-reg-list

The multi regs are derived from the single registers. Only list the
single registers in get-reg-list. This also makes the SBI extension
register listing consistent with the ISA extension register listing.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Haibo Xu <haibo1.xu@intel.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>