boards: px4_fmu-v5x restore rc.board_mavlink

- this was unintentionally removed in https://github.com/PX4/PX4-Autopilot/pull/16180

boards/px4/fmu-v5x/init/rc.board_mavlink

@@ -0,0 +1,10 @@
+#!/bin/sh
+#
+# board specific MAVLink startup script.
+#------------------------------------------------------------------------------
+
+if ver hwtypecmp V5Xa0 V5X91 V5Xa1
+then
+	# Start MAVLink on the UART connected to the mission computer
+	mavlink start -d /dev/ttyS4 -b 3000000 -r 290000 -m onboard_low_bandwidth -x -z
+fi