dt-bindings: reserved-memory: Support MTD/block device

Add bindings to allow MTD/block devices to be used in reserved-memory
regions using the "phram" (MTD in PHysical RAM) driver.

This allows things like partitioning to be specified via the existing
devicetree bindings.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220412135302.1682890-3-vincent.whitchurch@axis.com