usb: musb: add support for JZ4740 usb device controller

Add support for Ingenic JZ4740 USB Device Controller through a
specific musb glue layer.

JZ4740 UDC not being OTG compatible and missing some hardware
registers, this musb glue layer is written from scratch to be used in
gadget mode only and take silicon design specifics into account.

Signed-off-by: Apelete Seketeli <apelete@seketeli.net>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>