iio: adc: Add support for axp288 adc

Platform driver for X-Powers AXP288 ADC, which is a sub-device of the
customized AXP288 PMIC for Intel Baytrail-CR platforms. GPADC device
enumerates as one of the MFD cell devices. It uses IIO infrastructure
to communicate with userspace and consumer drivers.

Usages of ADC channels include battery charging and thermal sensors.

Based on initial work by:
Ramakrishna Pallala <ramakrishna.pallala@intel.com>

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>