usb: gadget: add endpoint capabilities flags

Introduce struct usb_ep_caps which contains information about capabilities
of usb endpoints - supported transfer types and directions. This structure
should be filled by UDC driver for each of its endpoints, and will be
used in epautoconf in new ep matching mechanism which will replace ugly
guessing of endpoint capabilities basing on its name.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>