MAINTAINERS: Add dt-bindings to mailbox entry

The MAINTAINER entry for the MAILBOX framework does not cover the
dt-bindings and as such Jassi is not among the recipients for such
patches.
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

MAINTAINERS

@@ -10860,6 +10860,7 @@ S:	Maintained
 F:	drivers/mailbox/
 F:	include/linux/mailbox_client.h
 F:	include/linux/mailbox_controller.h
+F:	include/dt-bindings/mailbox/
 F:	Documentation/devicetree/bindings/mailbox/
 
 MAILBOX ARM MHUv2