rpi-4: move/pin eth0 IRQs to CPU2

Move all eth0 IRQ processing to CPU2 instead of CPU1 which is the
default. CPU1 will continue handling all the USB interrupts which
also include USB-Ethernet adapter interrupts.

This significantly lowers the softirq load on CPU1 (from 99% to
less than 10%)

irqbalance could do the same and you can still run irqbalance
because the /etc/init.d/irq-eth-pin script will add the eth0 to
the ban list of irqbalance

See interrupt load with
  while true; do clear; cat /proc/interrupts; sleep 1; done

configs/rpi-4-files/etc/rc.d/S85irq-eth-pin

@@ -0,0 +1 @@
+../init.d/irq-eth-pin