riscv: Remove the 'riscv_' prefix of function name

Refactor the function name by removing the 'riscv_' prefix, it would be
better unless it could mix up with arch-independent functions.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>