This patch takes the software SPI implementation from extmod/machine_spi.c
and moves it to a dedicated file in drivers/bus/softspi.c. This allows the
SPI driver to be used independently of the uPy runtime, making it a more
general component.
Recent vendor SDKs ship libs with code in .text section, which previously
was going into .irom0.text. Adjust the linker script to route these
sections back to iROM (follows upstream change).
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
