ARM: dts: ls1021a: Add label to USB controllers

Add usb2 and usb3 labels to USB2 and USB3 controller device tree nodes,
for easier modification in board dts files.

Signed-off-by: Esben Haabendal <eha@deif.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>