net: ethernet: actions: Add Actions Semi Owl Ethernet MAC driver

Add new driver for the Ethernet MAC used on the Actions Semi Owl
family of SoCs.

Currently this has been tested only on the Actions Semi S500 SoC
variant.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>