boards: arm: nrf: Fix dts avoid_unnecessary_addr_size warn

We get the following warning when building nrf52810_pca10040:

	Warning (avoid_unnecessary_addr_size): unnecessary
	#address-cells/#size-cells without "ranges" or child
	"reg" property in /buttons

Remove #address-cells/#size-cells since they aren't needed for gpio_keys

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>