dm: sandbox: Enable of-platdata for sandbox_spl

Enable this feature so that we can use it for testing in sandbox_spl.

Signed-off-by: Simon Glass <sjg@chromium.org>

configs/sandbox_spl_defconfig

@@ -64,6 +64,7 @@ CONFIG_CMD_FS_GENERIC=y
 CONFIG_OF_CONTROL=y
 CONFIG_SPL_OF_CONTROL=y
 CONFIG_OF_HOSTFILE=y
+CONFIG_SPL_OF_PLATDATA=y
 CONFIG_NETCONSOLE=y
 CONFIG_SPL_DM=y
 CONFIG_REGMAP=y