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/Makefile b/drivers/adc/Makefile
index eb85b8b..cebf26d 100644
--- a/drivers/adc/Makefile
+++ b/drivers/adc/Makefile
@@ -7,3 +7,4 @@
 
 obj-$(CONFIG_ADC) += adc-uclass.o
 obj-$(CONFIG_ADC_EXYNOS) += exynos-adc.o
+obj-$(CONFIG_ADC_SANDBOX) += sandbox.o