dt-bindings: input: adc-keys bindings documentation

Dump adc-keys bindings documentation from Linux kernel source tree from
commit 698dc0cf9447 ("dt-bindings: input: adc-keys: clarify
description").

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