device-types: bcm2711-rpi-4-b: support booting rpi-sources
The current device-type of bcm2711-rpi-4-b works well with mainline Linux. But the rpi-sources does not boot with it. First it miss the cma size. Then without 8250.nr_uarts=1, the rpi-sources fail to open the console. But with 8250.nr_uarts=1, ttyS1 become ttyS0. Instead of hardcoding too many things, lets's permit to override extra_kernel_args.