Driver improvements (#535)

- New drivers:
  - SD SPI
  - spi_peripheral
  - touch_placeholder
  - display_placeholder
- Devicetree compiler:
  - Implement phandle-arrays
  - Implement device addresses
- Add placeholder drivers to all devices with a SPI display
- SPI driver: add `cs-pins` and set them to high on driver start
- FileSystem: add `file_system_set_owner()` and `file_system_get_owner()`
- File locking is now checking if the related `FileSystem` is part of a shared SPI bus
- Add `device_get_child_count()`
- SDMMC driver: Remove default of `slot` value
- Fix for Crowpanel Basic 3.5" display (add delay to booting)
- Fix for LilyGO T-HMI SD card mounting (delayed mounting by disabling it initially in the dts)

Documentation/README.md

@@ -27,6 +27,12 @@ idf.py flash monitor            # flash and monitor
 
 Device IDs are the folder names under `Devices/` (e.g. `lilygo-tdeck`, `m5stack-cores3`, `cyd-2432s028r`).
 
+### Devicetree
+
+A device implementation has a `.dts` file.
+The parser at `Buildscripts/DevicetreeCompiler/` converts DTS into C code.
+It's called from the `Firmware/` build process.
+
 ### Tests
 
 Tests use Doctest and run on simulator (POSIX) target only: