usb: gadget: add usb otg descriptor allocate and init interface

Allocate usb otg descriptor and initialize it according to gadget's otg
capabilities, if usb_otg_caps is not set, keep settings as current gadget
drivers. With this 2 new interfaces, gadget can use usb_otg_descriptor
for OTG 1.x, and usb_otg20_descriptor for OTG 2.0 or above, and otg
features can be decided by the combination of usb hardware property
and driver config.

Signed-off-by: Li Jun <jun.li@freescale.com>
Reviewed-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>