mtd: parsers: ofpart: support BCM4908 fixed partitions

Some devices use fixed partitioning with some partitions requiring some
extra logic. E.g. BCM4908 may have multiple firmware partitions but
detecting currently used one requires checking bootloader parameters.

To support such cases without duplicating a lot of code (without copying
most of the ofpart.c code) support for post-parsing callback was added.

BCM4908 support in ofpart can be enabled using config option and results
in compiling & executing a specific callback. It simply reads offset of
currently used firmware partition from the DT. Bootloader specifies it
using the "brcm_blparms" property.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210301105823.31032-1-zajec5@gmail.com