When we allocate space a buffer with some extra space for the FDT, adjust

When we allocate space a buffer with some extra space for the FDT, adjust
the size of the FDT to reflect the size of that buffer.  This prevents
an FDT overflow if the original FDT doesn't have enough space for the
additional properties that we add to it in our bootloader.

Fixes boot on the mcbin.
tested by bluhm@, ok patrick@