iio: adc: xilinx: support all platforms

Since the XADC logic can be used with standalone Xilinx FPGAs, this
driver can potentially be used with many different platforms, not just
the Zynq and MicroBlaze platforms this driver was allowed to be built
for. There should be no platform-specific code in this driver, so just
delete the platform dependency.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>