dt-bindings: mtd: make partitions doc a bit more generic

Currently the only documented partitioning is "fixed-partitions" but
there are more methods in use that we may want to support in the future.
Mention them and make it clear Fixed Partitions are just a single case.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Rob Herring <robh@kernel.org>