kernel: backport mtd support for subpartitions in DT

kernel: backport mtd support for subpartitions in DT

This is a new & warm feature that allows nesting partiitons in DT and
mixing their types (e.g. static vs. dynamic). It's very useful for
boards that have most partitions static but some of them require extra
parsing (e.g. a "firmware" partition).

It's required to successfully backport support for new devices using
that new syntax in their DT files.

Since brcm63xx has a custom alternative patch the upstream one is being
reverted for it. The plan is to make brcm63xx use the upstream
implementation.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 2a598bbaa3f75b7051c2453a6ccf706191cf2153)