mtd: partitions: add support for subpartitions

Some flash device partitions can be containers with extra subpartitions
(volumes). All callbacks are already capable of this additional level of
indirection.

This patch makes sure we always display subpartitions using a tree
structure and takes care of deleting subpartitions when parent gets
removed.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>