dm: rtc: sandbox: Enable real-time clock support

Enable real-time-clock support in sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 5de7fbe..759f53a 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -26,3 +26,4 @@
 CONFIG_SOUND=y
 CONFIG_CMD_SOUND=y
 CONFIG_SOUND_SANDBOX=y
+CONFIG_DM_RTC=y