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: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>