iio: adc: add MAX40080 current-sense amplifier driver

The MAX40080 is a bidirectional current-sense amplifier with an
integrated 12-bit ADC and an I2C/SMBus interface. It measures the
voltage across an external shunt resistor and the input bus voltage,
storing the results in an internal FIFO.

Add a direct-mode IIO driver exposing the current and voltage channels
with raw, scale and oversampling ratio attributes, a configurable
oversampling (digital averaging) ratio, and PEC-protected register
access. The current scale is derived from the shunt resistor value
described in the device tree.

The driver operates in single-measurement mode: each raw read triggers
an on-demand conversion via SMBus Quick Command and returns a matched
current/voltage pair. This avoids the latency and complexity of the
continuous FIFO mode while ensuring each read reflects the current
state. The two selectable current-sense ranges are exposed through
scale/scale_available.

Continuous FIFO buffering, threshold events and the alert interrupt are
intentionally left out of this initial submission and may be added
later.

Co-developed-by: Ciprian Hegbeli <ciprian.hegbeli@analog.com>
Signed-off-by: Ciprian Hegbeli <ciprian.hegbeli@analog.com>
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Reviewed-by: Siratul Islam <siratul.islam@linux.dev>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

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