sandbox: Add a test device that uses of-platdata

Start up the test devices. These print out of-platdata contents, providing a
check that the of-platdata feature is working correctly.

The device-tree changes are made to sandbox.dts rather than test.dts. since
the former controls the of-platdata generation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/sandbox_spl.h b/include/configs/sandbox_spl.h
index 7b5c3f3..ffc3098 100644
--- a/include/configs/sandbox_spl.h
+++ b/include/configs/sandbox_spl.h
@@ -8,6 +8,8 @@
 
 #include <configs/sandbox.h>
 
+#define CONFIG_SPL_BOARD_INIT
+
 #define CONFIG_SPL_DRIVERS_MISC_SUPPORT
 #define CONFIG_SPL_ENV_SUPPORT
 #define CONFIG_SPL_FRAMEWORK