fdt_support: Remove fdt_alloc_phandle() in favor of fdt_generate_phandle()

Commit f0921f50 ("fdt: Sync up to the latest libfdt") introduced
fdt_generate_phandle() in libfdt, making fdt_alloc_phandle() obsolete in
fdt_support.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: "hui.song" <hui.song_1@nxp.com>
Cc: Meenakshi Aggarwal <meenakshi.aggarwal@nxp.com>
Cc: Priyanka Jain <priyanka.jain@nxp.com>
Cc: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>