mtd: mtdpart: add a generic mtdparts-like parser

The current parser is very specific to U-Boot mtdparts implementation.
It does not use MTD structures like mtd_info and mtd_partition. Copy
and adapt the current parser in drivers/mtd/mtd-uclass.c (to not break
the current use of mtdparts.c itself) and write some kind of a wrapper
around the current implementation to allow other commands to benefit
from this parsing in a user-friendly way.

This new function will allocate an mtd_partition array for each
successful call. This array must be freed after use by the caller.
The given 'mtdparts' buffer pointer will be moved forward to the next
MTD device (if any, it will point towards a '\0' character otherwise).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>