test: add a simple test for the adc-keys button driver

Add adc-keys device to the sandbox/test.dts and connect it to the channel
#3 of the sandbox_adc driver. The default values sampled by sandbox_adc
driver determines that button3 and button4 are released and button5 is
pressed.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>