mtd: Update the way partitions are parsed

In case mtd_info's dev field is not populated (raw nand's case),
use the flash_node new field which reference the DT flash node where
can be found "partitions" node with "fixed-partitions" compatible.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>

Cc: Farhan Ali <farhan.ali@broadcom.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Marek Behun <marek.behun@nic.cz>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Wolfgang Denk <wd@denx.de>