auxdisplay: lcd2s DT binding doc

Add a binding doc for the modtronix lcd2s auxdisplay driver. It also
adds modtronix to the list of known vendor-prefixes.

Reviewed-by: Willy Tarreau <w@1wt.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>