dtb: am335x: Remove links and add more dts

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.