staging:iio: Allow to build SoC specific drivers when COMPILE_TEST is set

None of the SPEAr, LPC32XX or MXS ADC drivers have a compile time dependency on
their respective platform. So make it possible to build the drivers when
CONFIG_COMPILE_TEST is set. This makes it easier to compile test changes.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Roland Stigge <stigge@antcom.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>