Fixed mips_io_port_base build errors.

This patch has been sent on:
- 29 Sep 2007

Although mips_io_port_base is currently a part of IDE command, it is quite
fundamental for MIPS I/O port access such as in[bwl] and out[bwl]. So move
it to MIPS general part, and introduce `set_io_port_base()' from Linux.

This patch is triggered by multiple definition of `mips_io_port_base' build
error on gth2 (and tb0229 also needs this fix.)

board/gth2/libgth2.a(gth2.o): In function `log_serial_char':
/home/skuribay/devel/u-boot.git/board/gth2/gth2.c:47: multiple definition of `mips_io_port_base'
common/libcommon.a(cmd_ide.o):/home/skuribay/devel/u-boot.git/common/cmd_ide.c:712: first defined here
make: *** [u-boot] Error 1

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>