dt-bindings: mtd: physmap: Reuse the generic definitions

The memory mapped MTD devices also share a lot with all the other MTD
devices, so let's share the properties by referencing mtd.yaml. We can
then drop mentioning the properties, to the cost of mentioning the
possible "sram" node name prefix.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-11-miquel.raynal@bootlin.com