[GSoC] Adding RPi4 support to qemu-system-aarch64

[Afront]

Hi,

I would like to propose adding RPi4 to the list of supported machines 
for `qemu-system-aarch64` as my GSoC project, but I'm not sure if it's a 
suitable idea for GSoC since it might be too simple. In addition, there 
is already ongoing progress here 
<https://patchew.org/QEMU/20201018205551.1537927-1-f4...@amsat.org/>, 
but the last update is from a year and a half ago, and it is stated not 
complete as "more firmware properties" still have to be implemented.

Thanks,

Luke