usb: ateml: samd21: enable usb controller in board dts not soc

Since not all boards enable all devices, we typically have the SoC dtsi
file have a device marked with status = "disabled" and have the
board.dts explicitly enable with status = "ok".  Update it so USB on
Atmel SAMD21 work this way.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>