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/plat/fvp/platform.mk b/plat/fvp/platform.mk
index 8a33a60..ffed7e4 100644
--- a/plat/fvp/platform.mk
+++ b/plat/fvp/platform.mk
@@ -66,6 +66,7 @@
 				drivers/io/io_fip.c				\
 				drivers/io/io_memmap.c				\
 				drivers/io/io_semihosting.c			\
+				drivers/io/io_storage.c				\
 				lib/aarch64/xlat_tables.c			\
 				lib/semihosting/semihosting.c			\
 				lib/semihosting/aarch64/semihosting_call.S	\