Add support for ADA4355

[PIoandan]

PR Description

The ADA4355 is a complete, high performance, current input µModule. The 14-bit ADC converts the amplified voltage signal at a rate of up to 125 MSPS and outputs the digitized signals through two serial, low voltage differential signaling (LVDS) data lanes. The data clock output (DCO) operates at frequencies of up to 500 MHz and supports double data rate (DDR) operation.

This driver is also in the process of upstreaming.

PR Type

• Bug fix (a change that fixes an issue)
• New feature (a change that adds new functionality)
• Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• I have conducted a self-review of my own code changes
• I have tested the changes on the relevant hardware
• I have updated the documentation outside this repo accordingly (if there is the case)

[gastmaier]

edit: my comment looks into why it was lacking symbols, nuno suggestion is the correct solution.

Suggested change

	depends on SPI
	depends on SPI
	depends on CF_AXI_ADC

The driver uses axiadc_write and axiadc_read, exported from cf_axi_adc_core.c, enabled with CF_AXI_ADC.
So the driver depends on this symbol as well.
Without this change, it cannot be compiled as a module without blindly enabling symbols.
This is an issue in other drivers.

For curiosity, we have now a script that resolves the tree of depends in the KConfig based on a target:

$ python3 ci/symbols_depend.py drivers/iio/adc/ada4355.o
Symbols of touched files:
{'ADA4355'}
Resolved symbols:
{'PCI', 'IIO_BUFFER', 'HAS_IOMEM', 'HAVE_PCI', 'ADA4355', 'SPI', 'IIO'}

as committed, it lacks the CF_AXI_ADC symbol.

[nunojsa]

Why using the above and not the new IIO backend framework? That's what we want to use for all new drivers interfacing with the AXI ADC IP. This also makes it possible to upstream the driver.