mb/supermicro: restructure x11ssh-tf to represent a x11 board series

mb/supermicro: restructure x11ssh-tf to represent a x11 board series

Most of the X11 boards with socket LGA1151 are basically the same boards
with just some minor differences like different NICs (1 GbE, 10 GbE),
number of NICs / PCIe ports etc.

There are about 20 boards that can be added, if there is a community for
testing.

To be able to add more x11 boards easily like x11ssm (see CB:35427) this
restructures the x11ssh tree to represent a "X11 LGA1151 series". There
were multiple suggestions for the structure like grouping by series
(x10, x11, x...), grouping by chipset or by cpu family.

It turned out that there are some "X11 series" boards that are
completely different. Grouping by chipset or cpu family suffers from the
same problem. This is why finally we agreed on grouping by series and
socket ("X11 LGA1151 series").

The structure uses the common baseboard scheme, while there is no "real"
baseboard we know of. By checking images, comparing logs etc. we came to
the conclusion that Supermicro does have some base layout which is only
modified a bit for the different boards.

X11SSH-TF was moved to the variants/ folder with it's gpio.h. As we
expect the other boards to have mostly the same device tree, there is a
common devicetree that gets overridden by each variant's overridetree.

Besides that some very minor modifications happened (formatting, fixing
comments, ...) but not much.

Documentation is reworked in CB:35547

Change-Id: I8dc4240ae042760a845e890b923ad40478bb8e29
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35426
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>