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>
diff --git a/drivers/iio/adc/Kconfig b/drivers/iio/adc/Kconfig
index 11b048a..db2681b 100644
--- a/drivers/iio/adc/Kconfig
+++ b/drivers/iio/adc/Kconfig
@@ -127,6 +127,14 @@
 	help
 	  Say yes here to build support for Atmel AT91 ADC.
 
+config AXP288_ADC
+	tristate "X-Powers AXP288 ADC driver"
+	depends on MFD_AXP20X
+	help
+	  Say yes here to have support for X-Powers power management IC (PMIC) ADC
+	  device. Depending on platform configuration, this general purpose ADC can
+	  be used for sampling sensors such as thermal resistors.
+
 config EXYNOS_ADC
 	tristate "Exynos ADC driver support"
 	depends on ARCH_EXYNOS || (OF && COMPILE_TEST)