With a SPI flash that has more than 16MB, 32-bit addressing is required
rather than the standard 24-bit. This commit adds support for 32-bit
addressing so that the SPI flash commands (read/write/erase) are selected
automatically depending on the size of the address being used at each
operation.
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.
