arm: mvebu: generate DTBs for supported SoCs

Add the necessary dtb-$(CONFIG_...) entries so that "make dtbs"
generates the Device Tree Blobs that correspond to the selected mvebu
SoCs.
Signed-off-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: NArnd Bergmann <arnd@arndb.de>

arch/arm/mach-mvebu/Makefile.boot

 zreladdr-y := 0x00008000
+dtb-$(CONFIG_MACH_ARMADA_370_XP) += armada-370-db.dtb
+dtb-$(CONFIG_MACH_ARMADA_370_XP) += armada-xp-db.dtb