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>

arch/mips/lib/io.c

@@ -0,0 +1,12 @@
+/*
+ * (C) Copyright 2003
+ * Wolfgang Denk, DENX Software Engineering, wd@denx.de.
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+/*
+ * mips_io_port_base is the begin of the address space to which x86 style
+ * I/O ports are mapped.
+ */
+const unsigned long mips_io_port_base = -1;