ARM: dts: am3517.dtsi: Disable reference to OMAP3 OTG controller

The AM3517 has a different OTG controller location than the OMAP3,
which is included from omap3.dtsi.  This results in a hwmod error.
Since the AM3517 has a different OTG controller address, this patch
disabes one that is isn't available.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>