dm: test: Add a test for the system controller uclass

Add a test to confirm that we can access system controllers and find their
driver data.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 56d55ac..5535746 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -51,3 +51,4 @@
 CONFIG_DM_MMC=y
 CONFIG_LED=y
 CONFIG_LED_GPIO=y
+CONFIG_SYSCON=y