sandbox: Add a compatible string for spltest

At present this driver does not have a compatible string. For it to be
used with the coming of-platadata, it must have one. Update it
accordingly.

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

drivers/misc/spltest_sandbox.c

@@ -8,8 +8,14 @@
 #include <dm.h>
 #include <dt-structs.h>
 
+static const struct udevice_id sandbox_spl_ids[] = {
+	{ .compatible = "sandbox,spl-test", },
+	{}  /* sentinel */
+};
+
 U_BOOT_DRIVER(sandbox_spl_test) = {
 	.name	= "sandbox_spl_test",
 	.id	= UCLASS_MISC,
+	.of_match = sandbox_spl_ids,
 	.flags	= DM_FLAG_PRE_RELOC,
 };