dt-bindings: input: Add Cypress TT21000 touchscreen controller

dt-bindings: input: Add Cypress TT21000 touchscreen controller

Add the Cypress TrueTouch Generation 5 touchscreen device tree bindings
documentation. It can use I2C or SPI bus.
This touchscreen can handle some defined zone that are designed and
sent as button. To be able to customize the keycode sent, the
"linux,code" property in a "button" sub-node can be used.

Signed-off-by: Alistair Francis <alistair@alistair23.me>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221026114908.191472-3-alistair@alistair23.me
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>