RISC-V: Export riscv_cpuid_to_hartid_mask() API

The riscv_cpuid_to_hartid_mask() API should be exported to allow
building KVM RISC-V as loadable module.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>