ARM: multi_v7_defconfig: Enable support for the ADC thermal sensor

32-bit Amlogic Meson platforms are using a special ADC channel to read
the SoC temperature. Enable the "generic ADC thermal" driver so this
data can be used to cool the SoC for example by reduing the maximum CPU
and GPU frequencies temporarily.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>