[backport] iio: adc: ad4080: allow debugfs register access to both channels

[amiclaus]

PR Description

The dual-channel parts (AD4880/AD4883/AD4884) are built from two independent ADC dies, each with its own SPI chip select and register map. They are presented as a single IIO device because the FPGA interleaves both dies into one DMA stream, so only one direct_reg_access debugfs file is created and it can only reach channel A's regmap. Channel B's registers are therefore inaccessible, which blocks bring-up and debugging of the second die.

Select the target die through the debugfs register address: the low 8 bits are the register offset and the upper bits are the channel index (0x0nn selects channel A, 0x1nn channel B). A read returns the real register value of the selected die and each die is addressed independently; single-channel parts only accept channel 0.

Link: https://patch.msgid.link/20260826111613.14218-1-antoniu.miclaus@analog.com

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 compiled my changes, including the documentation
• I have tested the changes on the relevant hardware
• I have updated the documentation outside this repo accordingly
• I have provided links for the relevant upstream lore