fdt: Align the start of the livetree

Ensure that the block of memory used by live tree is aligned according to
the default for structures. This ensures that the root node appears at
the start of the block, so it can be used with free(), rather than being
4 bytes later in some cases.

This corrects a rather obscure bug in unflatten_device_tree().

Fixes: 8b50d526 ("dm: Add a function to create a 'live' device tree")

Signed-off-by: Simon Glass <sjg@chromium.org>