GPIO keys are properties of a board, not SoC, because SoC physically
does not have any keys or buttons.

This also fixes dtc W=1 build warning:

  sc9860.dtsi:688.13-714.5: Warning (simple_bus_reg): /soc/gpio-keys: missing or empty reg/ranges property
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20240401141128.98317-2-krzk@kernel.org