MIPS: move mips_io_port_base out of board.c

Move the definition of this variable out of arch/mips/lib/board.c in
preparation for allowing use of generic board on MIPS, which will lead
to this file not being compiled.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
diff --git a/arch/mips/lib/Makefile b/arch/mips/lib/Makefile
index fabeb83..88ef1c5 100644
--- a/arch/mips/lib/Makefile
+++ b/arch/mips/lib/Makefile
@@ -6,6 +6,8 @@
 #
 
 obj-y	+= board.o
+obj-y	+= io.o
+
 obj-$(CONFIG_CMD_BOOTM) += bootm.o
 
 lib-$(CONFIG_USE_PRIVATE_LIBGCC) += ashldi3.o ashrdi3.o lshrdi3.o