dt-bindings: mtd: move partition binding to its own file

Single partition binding is quite common and may be:
1. Used by multiple parsers
2. Extended for more specific cases

Move it to separated file to avoid code duplication.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>