usb: musb: ux500: add device tree probing support

This patch will allow ux500-musb to be probed and configured solely from
configuration found in Device Tree.

Cc: Rob Herring <rob.herring@calxeda.com>
Cc: linux-usb@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>