ad7124 adc driver refactoring

[trik]

Hi everyone. Writing some code to read a PT100 thermocouple via an AD7124 ADC, I was digging in the driver code and made some discoveries:

• The zephyr,differential property on channels is ignored. Bipolarity can be set using the bipolar-mask property on the device node
  

  zephyr/drivers/adc/adc_ad7124.c

  Line 418 in 74056a7

  new_cfg->props.bipolar = config->bipolar_mask & BIT(cfg->channel_id);

  
  The bipolar-mask property could be dropped in favor of the channel configuration be me more compliant to the ADC api

• Since AD7124 has two programmable excitation current sources, we could add two properties to set the current sources values (can be different) and enable the ADC_CONFIGURABLE_EXCITATION_CURRENT_SOURCE_PIN config

• AD7124 provides two GPIO output pins, it could be useful to have a GPIO driver.

If it sounds reasonable, i could take in charge.