dt-bindings: input: touchscreen: msg2638: Document keys support

Document optional linux,keycodes support.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vincent Knecht <vincent.knecht@mailoo.org>
Link: https://lore.kernel.org/r/20221116211622.2155747-2-vincent.knecht@mailoo.org


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