microblaze: Final support for statically linked DTB

If r7 is zero at kernel boot, or does not point to a valid DTB, then
we fall back to a DTB (assumed to be) linked statically in the kernel, instead
of blindly copying bogus cruft into the kernel DTB memory region

Signed-off-by: John Williams <john.williams@petalogix.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>