Add a simple Analog to Digital Converter device based button driver. This
driver binds to the 'adc-keys' device tree node.
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
