dt-bindings: memory: renesas,rpc-if: Miscellaneous improvements

  - Fix rejection of legitimate flash subnodes containing multiple
    compatible values,
  - Add missing list of required properties.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/d81b59a513c2a5204c8378b4a89cd07f97c46797.1627401508.git.geert+renesas@glider.be


Signed-off-by: Rob Herring <robh@kernel.org>