boards: arm: enable mailbox on NXP i.MX8M Mini EVK

Enable the messaging unit for NXP i.MX8M Mini EVK boards. This is a
necessary requirement to be able to run rpmsg examples later.

Signed-off-by: Matthias Fend <[email protected]>

Author: emfend

boards/arm/mimx8mm_evk/mimx8mm_evk.dts

@@ -28,3 +28,7 @@
 	status = "okay";
 	current-speed = <115200>;
 };
+
+&mailbox0 {
+	status = "okay";
+};