* Add PIO-based SoftwareSPI enabling SPI on any pins
The Raspberry Pi team has a working PIO-based SPI interface. Wrap it
to work like a hardware SPI interface, allowing SPI on any pin
combination.
Tested reading and writing an SD card using unmodified SD library.
* Add W5500 example
Good for testing, shows non-contiguous pin outs.
Fixes#1192
Uses DMA operations to avoid the need to bit-bang or busy wait for SPI
operations that might be very slow. Optional, adds new API calls to enable.
Simple example included.
