usb: gadget: pxa27x_udc: add devicetree support

Add support for device-tree device discovery. If devicetree is not
provided, fallback to legacy platform data "discovery".

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: devicetree@vger.kernel.org
Signed-off-by: Felipe Balbi <balbi@ti.com>