sandbox_flattree: enable button support

sandbox_flattree enables QCOM_PMIC_GPIO which now depends on BUTTON. As
a result the button tests now get run, but fail because the ADC and GPIO
button drivers aren't enabled.

Enable them to run the tests for sandbox_flattree.

Reported-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>