mvebu: fix initramfs/kernel image for CZNIC Turris Omnia

This adds DTB to kernel and that way makes it possible to easily boot
initramfs image and also kernel.

The sequence to boot initramfs on Omnia is then just:
  env set bootargs earlyprintk console=ttyS0,115200
  dhcp 0x1000000 192.168.1.1:openwrt-mvebu-cortexa9-cznic_turris-omnia-initramfs-kernel.bin
  bootz 0x1000000

Without this change kernel boot won't proceed and is stuck on "Starting
kernel".

Signed-off-by: Karel Kočí <karel.koci@nic.cz>
[fixed From: to match with SoB:]
Signed-off-by: Petr Štetiar <ynezz@true.cz>