dt-bindings: usb: Fix references to usb-hcd.yaml

usb-hcd.txt has been converted to YAML. Update references accordingly.

Fixes: 14ec072a

 ("dt-bindings: usb: Convert USB HCD generic binding to YAML")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20191003193132.17758-1-j.neuschaefer@gmx.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>