property for spi devices, although in the spigen case it's expected that
the speed will be overridden at runtime via the ioctl interface. A very
conservative 500khz speed is used (I've never seen a spi device that
couldn't run at 1mhz).
By adding spigen-rpi{2,-b}.dtso to fdt_overlays= in loader.conf, the fdt data
will set up the correct pinmux and device nodes to create a spigen(4) device
for each available chipselect pin.
Submitted by: Bob Frazier
Differential Revision: https://reviews.freebsd.org/D15067