rpi4: Enable GENET Ethernet controller

The Raspberry Pi 4 SoC features an integrated Gigabit Ethernet
controller, connected as a platform device.

Enable the new driver in the three applicable defconfigs, to allow
TFTP booting on the board.

Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>
[Andre: Add joined and 32-bit configs]
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>