mtd: parsers: add support for Sercomm partitions

This adds an MTD partition parser for the Sercomm partition table that
is used in some Beeline, Netgear and Sercomm routers.

The Sercomm partition map table contains real partition offsets, which
may differ from device to device depending on the number and location of
bad blocks on NAND.

Original patch (proposed by NOGUCHI Hiroshi):
Link: https://github.com/openwrt/openwrt/pull/1318#issuecomment-420607394



Signed-off-by: NOGUCHI Hiroshi <drvlabo@gmail.com>
Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220529110714.189732-1-csharper2005@gmail.com