The dtb was loaded at a memory address after the initial ramdisk. Thus a
large ramdisk would overwrite the dtb. Move it to "ramdisk_start - 64k".
64k should be enough for the device tree blob. Also the kernel
documentation arm/Booting suggests to put the dtb before the initial
ramdisk.

Signed-off-by: Michael Walle <michael@walle.cc>