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>

Documentation/devicetree/bindings/mailbox/sti-mailbox.txt

@@ -44,7 +44,7 @@ Optional properties
 Example:
 
 mailbox_test {
-	compatible	= "mailbox_test";
+	compatible	= "mailbox-test";
 	reg		= <0x[shared_memory_address], [shared_memory_size]>;
 	mboxes		= <&mailbox2 0 1>, <&mailbox0 2 1>;
 	mbox-names	= "tx",	"rx";