So far we used the legacy partition binding when fixing up the
mtd partition nodes. Change this to default to the new binding
with a "partitions" submode. Make this behaviour configurable
though: This creates a new of_binding device variable for
mtd devices. This can be set to:

- "new": Use the new partition binding (default)
- "legacy": Use the old partition binding
- "donttouch": Do not touch the partition node

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>