dt-bindings: input: Drop unneeded quotes

Cleanup bindings dropping unneeded quotes. Once all these are fixed,
checking for this can be enabled in yamllint.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # for mediatek,mt6779-keypad.yaml
Link: https://lore.kernel.org/r/20230320234718.2930154-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>