This adds PL011 UART support which is more plentiful.
It also:
* Fixes PI4 build by including .dtb files on the SD card.
* Enables the activity LED as the status LED on PI4 and CM4 I/O.
* Adds that LED as board.LED.
Fixes#5650 and progress on #5629
This targets the 64-bit CPU Raspberry Pis. The BCM2711 on the Pi 4
and the BCM2837 on the Pi 3 and Zero 2W. There are 64-bit fixes
outside of the ports directory for it.
There are a couple other cleanups that were incidental:
* Use const mcu_pin_obj_t instead of omitting the const. The structs
themselves are const because they are in ROM.
* Use PTR <-> OBJ conversions in more places. They were found when
mp_obj_t was set to an integer type rather than pointer.
* Optimize submodule checkout because the Pi submodules are heavy
and unnecessary for the vast majority of builds.
Fixes#4314
