dt-bindings: mediatek,mt6779-keypad: use unevaluatedProperties

dt-bindings: mediatek,mt6779-keypad: use unevaluatedProperties

writing-bindings.rst states:
> - If schema includes other schema (e.g. /schemas/i2c/i2c-controller.yaml) use
>   "unevaluatedProperties:false". In other cases, usually use
>   "additionalProperties:false".

All 3 properties from matrix-keymap.yaml are valid for the MediaTek keypad:
* keypad,num-rows and keypad,num-cols configure the KP_SEL register
* linux,keymap represents the (at most) 8x8 hardware matrix

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220720-mt8183-keypad-v2-2-6d42c357cb76@baylibre.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>