drivers: adc: adc_ad559x: add invernal vref value

Add configuration of internal refference voltage value so raw ADC
readings can be converted to mV utilizing adc_raw_to_millivolts()
ADC API function.

(cherry picked from commit 5523571)

Original-Signed-off-by: Lukasz Madej <l.madej@grinn-global.com>
GitOrigin-RevId: 5523571
Change-Id: Icaa5cbd529dc5aa4d60a6f940a5db05e23987aad
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5488596
Commit-Queue: Dawid Niedźwiecki <dawidn@google.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Tested-by: Dawid Niedźwiecki <dawidn@google.com>
Reviewed-by: Dawid Niedźwiecki <dawidn@google.com>

drivers/adc/adc_ad559x.c

@@ -21,6 +21,7 @@ LOG_MODULE_REGISTER(adc_ad559x, CONFIG_ADC_LOG_LEVEL);
 #define AD559X_ADC_RD_POINTER      0x40
 
 #define AD559X_ADC_RESOLUTION 12U
+#define AD559X_ADC_VREF_MV 2500U
 
 struct adc_ad559x_config {
 	const struct device *mfd_dev;
@@ -257,6 +258,7 @@ static const struct adc_driver_api adc_ad559x_api = {
 #ifdef CONFIG_ADC_ASYNC
 	.read_async = adc_ad559x_read_async,
 #endif
+	.ref_internal = AD559X_ADC_VREF_MV,
 };
 
 #define ADC_AD559X_DEFINE(inst)                                                                    \