MIPS: Loongson: Cleanup the serial port support

To share the same kernel image amon different machines we have added the
machtype command line support.

In the old serial port implementation the UART base address is hardcoded as
a macro in machine.h which breaks with machtype, so change that to discover
the address dynamically.  Also move the initialization of the UART base
address to uart_base.c to avoid remapping twice for early_printk.c and
serial.c.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/581/
Patchwork: http://patchwork.linux-mips.org/patch/682/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>