RISC-V: Export sbi_get_mvendorid() and friends

The sbi_get_mvendorid(), sbi_get_marchid(), and sbi_get_mimpid()
can be used by KVM module so let us export these functions.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>