dt-bindings: arm: Convert Gemini boards to YAML

This removes the old plaintext Gemini binding and replace it
with a YAML schema, adding some new boards in the process.
While we are at it, add the missing vendors to the vendor
prefix file.

Drop the overly deliberate description of subnodes and the big
example from the old document. Keep the elaborate description.

I noticed that "wiliboard" is not a real vendor, the vendor
is named "wiligear" so deprecated this and replaced with the
proper vendor.

Cc: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210723152356.1874088-1-linus.walleij@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>