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/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 67ae99b..94c8e68 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -65,3 +65,5 @@
 CONFIG_UT_ENV=y
 CONFIG_REMOTEPROC_SANDBOX=y
 CONFIG_CMD_REMOTEPROC=y
+CONFIG_ADC=y
+CONFIG_ADC_SANDBOX=y