mtd: Add flash_node in struct mtd_info

mtd: Add flash_node in struct mtd_info

Currently, add_mtd_partitions_of() can be used only if dev field of
mtd_info struct is populated. It's the case, for example, for a spi nor
flash, which has a DT compatible "jedec,spi-nor" and an associated
device. mtd->dev is populated in spi_nor_scan().

But in case of a raw nand node, mtd_info's dev field can't be populated
as flash node has no compatible, so no associated device.
add_mtd_partitions_of() can't be used to parse "partitions" subnode.

To remove this constraint, add an ofnode field in mtd_info struct
which reference the DT flash node. This new field is populated by
nand_scan_tail(). This new field will be used by add_mtd_partitions_of()
to parse the flash node for "partitions" defined in DT.

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>