memory: renesas-rpc-if: Add missing static keyword

Fix the below sparse warnings:
 symbol 'rpcif_impl' was not declared. Should it be static?
 symbol 'xspi_impl' was not declared. Should it be static?

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505072013.1EqwjtaR-lkp@intel.com/


Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20250507162146.140494-1-biju.das.jz@bp.renesas.com


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