MIPS: Loongson: Cleanups of serial port support

This patchs uses a loongson_uart_base variable instead of the
uart_base[] array and adds a new kernel option to avoid to compile
uart_base.c all the time, which will save a little bit of memory for us.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
http://patchwork.linux-mips.org/patch/727/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>