[Feature Request] Add Raspberry Pi 4 memory map / Detection

Add in ability to probe the raspberry pi 4 to see how much memory it has. 
This is currently partially stubbed out in [src/arch/rpi4/bringup.c](https://github.com/primis/Apollo/blob/main/src/arch/rpi4/bringup.c) at `free_memory_get_megs()` but requires also ranges to be defined.