iio: adc: ad4080: allow debugfs register access to both channels

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.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

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