MIPS: Loongson: Add serial port support

This patch add serial port support for all of the existing loongson
family machines. most of the board specific part are put in serial.c,
and the base address of the serial ports are defined as macros in
machine.h for sharing it between serial.c and early_printk.c

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Cc: yanh@lemote.com
Cc: huhb@lemote.com
Cc: Zhang Le <r0bertz@gentoo.org>
Cc: zhangfx@lemote.com
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>