iio: adc: Kconfig: add COMPILE_TEST dep for berlin2-adc

Otherwise most build checks will omit this driver from a compile-test due
to it's dependency only on the BERLIN_ARCH symbol.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210926192642.4051329-2-aardelean@deviqon.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>