arm: dts: imx6ul: add usbotg aliases

This is needed to make the UMS command work again as it fails with the
following error:

BIOS> ums 0 mmc 0
UMS: LUN 0, dev 0, hwpart 0, sector 0x0, count 0x748000
g_dnl_register: failed!, error: -19
ERROR: g_dnl_register failed
at cmd/usb_mass_storage.c:179/do_usb_mass_storage()

That's because usb_setup_ehci_gadget() function is looking for the usb
device using the req_sed number.
This change makes the usb device have a req_seq number and the UMS
command work again:

BIOS> ums 0 mmc 0
UMS: LUN 0, dev 0, hwpart 0, sector 0x0, count 0x748000
CTRL+C - Operation aborted

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>