arm: dts: imx6qdl-u-boot: Alias usb0 to usbotg

All i.mx6 boards seems to have moved to DM_USB, however gadget support
for mx6 is still pre-DM as CI_UDC isn't converted yet. To make this work
the usb otg controller used for gadgets needs to be usb number 0.
Add an alias for this directly in the main u-boot mx6qdl dtsi so it
doesn't need to be done for each board separately.

This fixes regressions wrt. usb gadget functionality in several boards
that have gadget functions enabled in their config, but no usb0 alias in
their device-tree.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>