realtek: split TP-Link SG2000 series devicetree

realtek: split TP-Link SG2000 series devicetree

The TP-Link TL-SG2008, TL-SG2008P, and TL-SG2210P use the same board.
The main difference is that some footprints are not populated in the
lower-end models. To model this with minimal duplication, move the
devicetree to a common dtsi, leaving out just the board name.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
[remove port relabelling from commit message, already merged with commit
 18a2b29aa1c9 ("realtek: tl-sg2008p: fix labeling of lan ports")]
Signed-off-by: Sander Vanheule <sander@svanheule.net>