arm64: dts: apple: j274: Expose PCI node for the Ethernet MAC address

At the moment, all the Minis running Linux have the same MAC
address (00:10:18:00:00:00), which is a bit annoying.

Expose the PCI node corresponding to the Ethernet device, and
declare a 'local-mac-address' property. The bootloader will update
it (m1n1 already has the required feature). And if it doesn't, then
the default value is already present in the DT.

This relies on forcing the bus number for each port so that the
endpoints connected to them are correctly numbered (and keeps dtc
quiet).

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>