dts: bindings: Move zephyr,memory-region property definition

Move it to a common file named `zephyr,memory-common.yaml`, which
replaces `zephyr,memory-attr.yaml` and takes its contents as well.

This is so that another binding, e.g., `vnd,memory-region`, can support
being combined with the `zephyr,memory-region` binding like so:

  node@deadbeef {
    compatible = "vnd,memory-region", "zephyr,memory-region";
    zephyr,memory-region = "NAME";
    ...
  };

To allow this, edtlib would require `vnd,memory-region` to include the
property definitions from this new common file.

The same can't be done by including `zephyr,memory-region.yaml` directly
because that file marks the property in question as always required, and
it shouldn't be required whenever the `vnd,memory-region` compatible is
used on its own.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>