Kernel commit 0782e8572c ("ARM: dts: Probe am335x musb with ti-sysc")
which we pulled in during the v2020.02.0 dts/ sync moved the USB nodes
to be under a ti-sysc bus instead of ti,am33xx-usb.

This new probe order broke am335x USB under barebox, because the MUSB
drivers couldn't cope with the now different device probe order.

Pepper some -EPROBE_DEFER around to make USB work again.

Fixes: 574eed3f6f ("dts: update to v5.5-rc1")
Reported-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>