Documentation: omap-usb-host: fix OMAP OHCI/EHCI file names

OMAP related files are actually named ehci-omap.txt and ohci-omap3.txt.

Also add full path to ohci-omap3.txt.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Rob Herring <robh@kernel.org>