configs: khadas-vim3: enable ADC device support

Analog to Digital Converter device (Meson SARADC) will be used for
probing 'Function' button state.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
[narmstrong: also updated khadas-vim3l_defconfig]
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>