Add memory region for IO

memory.x

@@ -1,8 +1,9 @@
 MEMORY
 {
-  INST   : ORIGIN = 0x00000000, LENGTH = 256K
+  INST  : ORIGIN = 0x00000000, LENGTH = 256K
-  CONST  : ORIGIN = 0x40000000, LENGTH = 16K
+  IO    : ORIGIN = 0x20000000, LENGTH = 1K
-  RAM    : ORIGIN = 0x80000000, LENGTH = 16K
+  CONST : ORIGIN = 0x40000000, LENGTH = 16K
+  RAM   : ORIGIN = 0x80000000, LENGTH = 16K
 }
 
 REGION_ALIAS("REGION_TEXT", INST);