sunxi: R40: add gigabit ethernet devicetree node

Add a device tree node for the Allwinner R40/V40 GMAC gigabit
ethernet interface.
The R40 SoC does not use the syscon register for GMAC settings.
The gigabit ethernet interface can only be routed to a fixed set of
pins.
Updated to match the Linux kernel's device tree.

Signed-off-by: Lothar Felten <lothar.felten@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@openedev.com>