dt-bindings: mtd: add YAML schema for the generic MTD bindings

Convert MTD's common.txt to mtd.yaml.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210424110608.15748-3-michael@walle.cc