mips: ath79: dts: Add ethernet MAC nodes for ar933x

Add node for both ethernet controllers in the ar933x.
The PHY is attached only to the first ethernet controller.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Wills Wang <wills.wang@live.com>