microblaze: enable building all dtbs

Enable the 'dtbs' target for microblaze. As microblaze only has one dts
file, always enable it.

Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Rob Herring <robh@kernel.org>

arch/microblaze/boot/dts/Makefile

 # SPDX-License-Identifier: GPL-2.0
 #
 
+dtb-y := system.dtb
+
 ifneq ($(DTB),)
 obj-y += linked_dtb.o