arm64: dts: renesas: rzg3s-smarc: Add gpio keys

RZ SMARC Carrier II board has 3 user buttons called USER_SW1, USER_SW2,
USER_SW3. Add a DT node in device tree to propertly instantiate the
gpio-keys driver for these buttons.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20231227130810.2744550-1-claudiu.beznea.uj@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>