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/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

arch/mips/lib/board.c

@@ -27,12 +27,6 @@ ulong monitor_flash_len;
 
 static char *failed = "*** failed ***\n";
 
-/*
- * 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;
-
 int __board_early_init_f(void)
 {
 	/*

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;