dt-bindings: input: atmel,maxtouch: add linux,keycodes

dt-bindings: input: atmel,maxtouch: add linux,keycodes

In some configurations the touch controller can support the touch keys.
Document the linux,keycodes property that enables those keys and
specifies the keycodes that should be used to report the key events.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: André Apitzsch <git@apitzsch.eu>
Link: https://lore.kernel.org/r/20230407-atmel_keys-v2-1-92446a4343cb@apitzsch.eu
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>