mtd: add support for parsing partitions defined in OF

Add support for parsing partitions defined in device-trees via the
`partitions` node with `fixed-partitions` compatible.

The `mtdparts`/`mtdids` mechanism takes precedence. If some partitions
are defined for a MTD device via this mechanism, the code won't register
partitions for that MTD device from OF, even if they are defined.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Heiko Schocher <hs@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>