Move IO storage source to drivers directory

Move the remaining IO storage source file (io_storage.c) from the
lib to the drivers directory. This requires that platform ports
explicitly add this file to the list of source files.

Also move the IO header files to a new sub-directory, include/io.

Change-Id: I862b1252a796b3bcac0d93e50b11e7fb2ded93d6
diff --git a/Makefile b/Makefile
index fcbf773..89e882a 100644
--- a/Makefile
+++ b/Makefile
@@ -102,7 +102,6 @@
 				lib/aarch64/misc_helpers.S		\
 				lib/aarch64/xlat_helpers.c		\
 				lib/stdlib/std.c			\
-				lib/io_storage.c			\
 				plat/common/aarch64/platform_helpers.S
 
 BUILD_BASE		:=	./build
@@ -180,6 +179,7 @@
 				-Iinclude/common		\
 				-Iinclude/drivers		\
 				-Iinclude/drivers/arm		\
+				-Iinclude/drivers/io		\
 				-Iinclude/lib			\
 				-Iinclude/lib/aarch64		\
 				-Iinclude/plat/common		\