dm: adc: add Exynos54xx compatible ADC driver

This commit adds driver for Exynos54xx ADC subsystem.

The driver is implemented using driver model, amd provides
ADC uclass's methods for ADC single channel operations:
- adc_start_channel()
- adc_channel_data()
- adc_stop()

The basic parameters of ADC conversion, are:
- sample rate: 600KSPS
- output the data as average of 8 time conversion

ADC features:
- sample rate: 600KSPS
- resolution: 12-bit
- channels: 10 (analog multiplexer)

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
diff --git a/drivers/adc/Kconfig b/drivers/adc/Kconfig
index b6e226a..223b65e 100644
--- a/drivers/adc/Kconfig
+++ b/drivers/adc/Kconfig
@@ -10,3 +10,12 @@
 	  - methods for get Vdd/Vss reference Voltage values with polarity
 	  - support supply's phandle with auto-enable
 	  - supply polarity setting in fdt
+
+config ADC_EXYNOS
+	bool "Enable Exynos 54xx ADC driver"
+	help
+	  This enables basic driver for Exynos ADC compatible with Exynos54xx.
+	  It provides:
+	  - 10 analog input channels
+	  - 12-bit resolution
+	  - 600 KSPS of sample rate