usb: gadget: configfs: OS Extended Properties descriptors support

Add handling of OS Extended Properties descriptors from configfs interface.
One kind of "OS Descriptors" are "Extended Properties" descriptors, which
need to be specified per interface or per group of interfaces described
by an IAD. This patch adds support for creating subdirectories
in interface.<n> directory located in the function's directory.
Names of subdirectories created become names of properties.
Each property contains two attributes: "type" and "data".
The type can be a numeric value 1..7 while data is a blob interpreted
depending on the type specified.
The types are:
1 - unicode string
2 - unicode string with environment variables
3 - binary
4 - little-endian 32-bit
5 - big-endian 32-bit
6 - unicode string with a symbolic link
7 - multiple unicode strings

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/include/linux/usb/composite.h b/include/linux/usb/composite.h
index 9c3903d..7373203 100644
--- a/include/linux/usb/composite.h
+++ b/include/linux/usb/composite.h
@@ -64,6 +64,7 @@
  * @name: Extended Property name
  * @data_len: Length of Extended Property blob (for unicode store double len)
  * @data: Extended Property blob
+ * @item: Represents this Extended Property in configfs
  */
 struct usb_os_desc_ext_prop {
 	struct list_head	entry;
@@ -72,6 +73,7 @@
 	char			*name;
 	int			data_len;
 	char			*data;
+	struct config_item	item;
 };
 
 /**
@@ -82,6 +84,7 @@
  * @ext_prop_count: Number of Extended Properties
  * @opts_mutex: Optional mutex protecting config data of a usb_function_instance
  * @group: Represents OS descriptors associated with an interface in configfs
+ * @owner: Module associated with this OS descriptor
  */
 struct usb_os_desc {
 	char			*ext_compat_id;
@@ -90,6 +93,7 @@
 	int			ext_prop_count;
 	struct mutex		*opts_mutex;
 	struct config_group	group;
+	struct module		*owner;
 };
 
 /**