fdt_support: refactor with fdt_find_or_add_subnode helper func

Some functions in fdt_support.c do the same routine:
search a node with a given name ("chosen", "memory", etc.)
or newly create it if it does not exist.

So this commit makes that routine to a helper function.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>