libfdt: Add fdt_header_size()

We have a couple of places within libfdt and its tests where we need to
find the size of the header, based on the version.  Add a helper function
for it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>