ARM: DTS: am335x-evm: Use USB0 in peripheral mode

This USB port is mainly used for RNDIS and DFU. To be able to use it with
DM_USB and DM_USB_GADGET, we need to provide a dr_mode value in the DTS.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>