fdt: Rename fdt_create_phandle to fdt_set_phandle

The old fdt_create_phandle didn't actually create a phandle it just
set one.  We'll introduce a new helper that actually does creation.

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