dt-bindings: mtd: update references from partition.txt to mtd.yaml

Commit f902baa9 ("dt-bindings: mtd: Remove useless file about
partitions") removed the file partition.txt. Hence, in this commit, the
lines mentioning this file are updated to reference mtd.yaml, which now
includes partition{,s}.yaml.

Signed-off-by: Nayab Sayed <nayabbasha.sayed@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240208-partition-txt-v1-1-4398af3b7bb2@microchip.com