net: sun8i-emac: support new pinctrl DT bindings

The Linux kernel driver for the Allwinner pin controller gained support
for generic properties, which are now also used in the DTs.
The sun8i-emac Ethernet driver for new Allwinner MACs reads the pins from
the DT, but so far only supported the old binding.
Update the parsing routine to cope with both the old and new bindings,
so that the newer DTs can be used with U-Boot and its Ethernet driver.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>