common: fdt_support: add support for "partitions" subnode to fdt_fixup_mtdparts()

Listing MTD partitions directly in the flash mode has been deprecated
for a while for kernel Device Trees. Look for a node "partitions" in the
found flash nodes and use it instead of the flash node itself for the
partition list when it exists, so Device Trees following the current
best practices can be fixed up.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Reviewed-by: Simon Glass <sjg@chromium.org>