ARM: dts: bcm2711-rpi-4: Enable GENET support

This enables the Gigabit Ethernet support on the Raspberry Pi 4.
The defined PHY mode is equivalent to the default register settings
in the downstream tree.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>