usb: gadget: f_hid: add dev to configfs

Even if the /dev/hidg* chardev is automatically created, one
has to guess which one belongs to which function. In the case
of multiple HID functions, or maybe even multiple peripherals,
this becomes difficult.

Add the dev (with major and minor number) to configfs to allow
looking up (or even creating) the right device node for each
function. This file is read-only.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>