net: Add ag7xxx driver for Atheros MIPS

Add ethernet driver for the AR933x and AR934x Atheros MIPS machines.
The driver could be easily extended to other WiSoCs.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Wills Wang <wills.wang@live.com>
[fixed Kconfig dependency]
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>