MIPS: Loongson: Move prom_argc and prom_argv into prom_init_cmdline()

prom_argc and prom_argv are only used by prom_init_cmdline(), move them
into the function.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: yanh@lemote.com
Cc: huhb@lemote.com
Cc: zhangfx@lemote.com
Patchwork: http://patchwork.linux-mips.org/patch/825/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>