dt-bindings: mediatek,mt6779-keypad: add mediatek,keys-per-group

The MediaTek keypad has 2 modes of detecting key events:
* single key: each (row, column) can detect one key
* double key: each (row, column) is a group of 2 keys

With double key, two keys are physically wired to one (row, column) pin.
These keys are in the same "group".

Multiple keys in the same group reduces the number of pins which
minimizes cost.

Add a keys-per-group property to describe this.

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-v2-3-6d42c357cb76@baylibre.com


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