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>