[MIPS] Work around bad code generation for <asm/io.h>.

If a call to set_io_port_base() was being followed by usage of
mips_io_port_base in the same function gcc was possibly using the old
value due to some clever abuse of const.  Adding a barrier will keep
the optimization and result in correct code with latest gcc.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>