ARM: dts: am335x-igep0033: use phandles for USB and DMA refs

Use phandles instead of unit adresses to reference usb and dma nodes.
This makes the DT more robust and readable.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Cc: Enric Balletbo i Serra <eballetbo@iseebcn.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>