dt-bindings: input: add GPIO charlieplex keypad

Add DT bindings for GPIO charlieplex keypad.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://patch.msgid.link/20260312180304.3865850-4-hugo@hugovil.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

[ upstream commit: 0d64bee764847a488ac33be8ec61b4ae7828f8f1 ]