usb:udc:samsung Add functions for storing private gadget data in UDC driver

This commit adds support for storing private data to Samsung's UDC
driver. This data is afterward used by usb gadget.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>