The Broadcom chips apparently don't have an RTC, so this PR uses the
timer to add dummy RTC responses and allow resonable results
from time.localtime() as well as setting the time and date while
the board is powered.
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
