ARM: dts: dra7: add labels to DWC3 nodes

by adding labels to DWC3 nodes, it's far easier
for boards to reference them.

Signed-off-by: Felipe Balbi <balbi@ti.com>
[tony@atomide.com: updated for otg 4 move to dra74x.dtsi]
Signed-off-by: Tony Lindgren <tony@atomide.com>