sandbox: add ADC driver

This commit adds implementation of Sandbox ADC device emulation.
The device provides:
- single and multi-channel conversion
- 4 channels with predefined conversion output data
- 16-bit resolution

Signed-off-by: Przemyslaw Marczak <p.marczak@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 223b65e..e5335f7 100644
--- a/drivers/adc/Kconfig
+++ b/drivers/adc/Kconfig
@@ -19,3 +19,12 @@
 	  - 10 analog input channels
 	  - 12-bit resolution
 	  - 600 KSPS of sample rate
+
+config ADC_SANDBOX
+	bool "Enable Sandbox ADC test driver"
+	help
+	  This enables driver for Sandbox ADC device emulation.
+	  It provides:
+	  - 4 analog input channels
+	  - 16-bit resolution
+	  - single and multi-channel conversion mode