iio: adc: add ADC driver for the TI LMP92064 controller

iio: adc: add ADC driver for the TI LMP92064 controller

The TI LMP92064 is a dual 12 Bit ADC connected via SPI.
The two channels are intended for simultaneous measurements of the voltage
across- and current through a load to allow accurate instantaneous power
measurements.
The driver does not yet take advantage of this feature, as buffering is not
yet implemented.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Link: https://lore.kernel.org/r/20221128133503.1355898-2-l.goehrs@pengutronix.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>