button: add missing ADC dependency for BUTTON_ADC

The BUTTON_ADC symbol guards the compilation of button-adc driver whose
name very well makes it explicit that it requires ADC support to be
enabled.

Fix build issue of button-adc driver when ADC support isn't enabled by
making sure it cannot be built without ADC support.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>