dt-bindings: input: touchscreen: Add Hynitron cstxxx

Add documentation for the Hynitron cstxxx touchscreen bindings.
Hynitron makes a series of touchscreen controllers, however for
now this is expected to only be compatible with the cst3xx series,
specifically the CST340, CST348, and CST356.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221028202636.14341-3-macroalpha82@gmail.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>