iio: adc: adi-axi-adc: add platform dependencies

Being this device a soft core, it's only supported on some/specific
platforms. Hence add proper dependencies for the supported platforms.
Also add COMPILE_TEST to increase the build coverage.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240604-dev-axi-adc-kconfig-v1-1-cfb725606b8e@analog.com


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