net: sun8i-emac: Add support for fixed-link phy

Make the "phy-handle" property optional, which allows support
for a fixed-link phy configuration.

Thus if the "phy-handle" is present in a DT, then driver will work as
before. Otherwise, phyaddr initialization will not be necessary,
as it is not needed in case of a fixed-link config.

Signed-off-by: Maksim Kiselev <bigunclemax@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>