fdt: Fix fdt padding issue for initrd mem_rsv

Its possible that we end up with a device tree that happens to be a
particular size that after we call fdt_resize() we don't have any
space left for the initrd mem_rsv.

Fix this be adding a second mem_rsv into the size calculation.  We
had one to cover the fdt itself and we have the potential of adding
a second for the initrd.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>