The links were to cope with the switch to upstream dts.
We don't need them anymore.
While here add the rest of the beaglebone family dts as u-boot is common
on all those boards and load the dtb based on the product name.
This just miss the pocketbeagle variant as it's not yet in sys/gnu/dts but
will be with the Linux 4.18 dts import.
default, and the fewer changes relative to the upstream u-boot the
better.
Add compatibility links for the old names.
Add dts file for BeagleBone Green while we're here.
