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>