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

drivers/iio/adc/Kconfig

@@ -354,7 +354,7 @@ config BCM_IPROC_ADC
 
 config BERLIN2_ADC
 	tristate "Marvell Berlin2 ADC driver"
-	depends on ARCH_BERLIN
+	depends on ARCH_BERLIN || COMPILE_TEST
 	help
 	  Marvell Berlin2 ADC driver. This ADC has 8 channels, with one used for
 	  temperature measurement.