dt-bindings: memory: renesas,rpc-if: Document R-Car V4M support

Document support for the SPI Multi I/O Bus Controller (RPC-IF) in the
Renesas R-Car V4M (R8A779H0) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/263d6626fd4fa51b175b5c7a53e6a363e2c91519.1708354280.git.geert+renesas@glider.be


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>