correct spigen-rpi2.dtso, add spigen-rpi3.dtso

[bobf_mrp3.com]

Summary

spigen-rpi2.dtso had an entry for CS=2 that actually does not work, so it has been removed. it now looks exactly like spigen-rpi-b.dtso except for the compatibility string.

Also adding spigen-rpi3.dtso with a compatibility string for the RPi 3.

Test Plan

verify operation on all 3 platforms, RPi 1 model , RPi 2, RPi 3

(verified on RPi 2 by copying the new as-built overlay and booting on slightly earlier kernel)

[bobf_mrp3.com]

cosmetic things, added the Makefile line to build the rpi3 patch

[db]

Looks good to me. It would be good to get this into -12 release

[vince.bsd_hightek.org]

We tested the spigen-rpi3.dtso overlay and it seems to work fine.

[bobf_mrp3.com]

is it too late to commit this to 12?

[bobf_mrp3.com]

I forgot to mention, someone suggested modifying everything so that only a single overlay is used for all 3 RPi versions. It would require that the compat strings in the single overlay include all 3 of the driver specs. In the past when I have attempted multiple compat strings, the overlay wouldn't load. It is possible I was doing something else wrong, however. Not sure if this should become a blocking issue or not.