mtd: partitions: use DT info for parsing partitions with "compatible" prop

So far only flash devices could be described in DT regarding partitions
parsing. That could be done with "partitions" subnode and a proper
"compatible" string.

Some devices may use hierarchical (multi-level) layouts and may mix used
layouts (fixed and dynamic). Describing that in DT is done by specifying
"compatible" for DT-represented partition plus optionally more
properties and/or subnodes.

To support such layouts each DT partition has to be checked for
additional description.

Please note this implementation will work in parallel with support for
partition type specified for non-DT setups. That already works since
commit 1a0915be

 ("mtd: partitions: add support for partition
parsers").

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>