mips/bootinfo:correct some comments of fw_arg

from arch/mips/kernel/head.S we know that use a0~a3 for fw_arg0~fw_arg3
there is some code from head.S:
	LONG_S		a0, fw_arg0		# firmware arguments
	LONG_S		a1, fw_arg1
	LONG_S		a2, fw_arg2
	LONG_S		a3, fw_arg3
Signed-off-by: xiaochuan mao <maoxiaochuan@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/bootinfo.h

@@ -107,7 +107,7 @@ extern void (*free_init_pages_eva)(void *begin, void *end);
 extern char arcs_cmdline[COMMAND_LINE_SIZE];
 
 /*
- * Registers a0, a1, a3 and a4 as passed to the kernel entry by firmware
+ * Registers a0, a1, a2 and a3 as passed to the kernel entry by firmware
  */
 extern unsigned long fw_arg0, fw_arg1, fw_arg2, fw_arg3;