mailbox: mailbox-test: fix the compatible string

Underscores are usually forbidden in the compatible strings. So lets
remove it before the first users of this is seen.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mailbox-test.c

@@ -342,7 +342,7 @@ static int mbox_test_remove(struct platform_device *pdev)
 }
 
 static const struct of_device_id mbox_test_match[] = {
-	{ .compatible = "mailbox_test" },
+	{ .compatible = "mailbox-test" },
 	{},
 };