Add spigen(4) fdt data overlays for RPI-B, RPI-2.

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