dt-bindings: net: Add Actions Semi Owl Ethernet MAC binding

Add devicetree binding for the Ethernet MAC present on the Actions
Semi Owl family of SoCs.

For the moment advertise only the support for the Actions Semi S500 SoC
variant.

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