ColdFire: Fix missing _IO_BASE which caused compile error

The compile error was caused by a recent patch. Affected platforms -
M5253DEMO.h, M5253EVBE.h, and M54455EVB.h. Adding the _IO_BASE
automatically defined to 0 in asm-m68k/io.h if it isn't set in
platform configuration file.

Signed-off-by: TsiChung Liew <tsicliew@gmail.com>

include/asm-m68k/io.h

@@ -28,6 +28,10 @@
 
 #include <asm/byteorder.h>
 
+#ifndef _IO_BASE
+#define _IO_BASE 0
+#endif
+
 #define __raw_readb(addr) (*(volatile u8 *)(addr))
 #define __raw_readw(addr) (*(volatile u16 *)(addr))
 #define __raw_readl(addr) (*(volatile u32 *)(addr))