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>

Author: bijudas

drivers/memory/renesas-rpc-if.c

@@ -1063,7 +1063,7 @@ static void rpcif_remove(struct platform_device *pdev)
 	platform_device_unregister(rpc->vdev);
 }
 
-struct rpcif_impl rpcif_impl = {
+static const struct rpcif_impl rpcif_impl = {
 	.hw_init = rpcif_hw_init_impl,
 	.prepare = rpcif_prepare_impl,
 	.manual_xfer = rpcif_manual_xfer_impl,
@@ -1072,7 +1072,7 @@ struct rpcif_impl rpcif_impl = {
 	.status_mask = RPCIF_CMNSR_TEND,
 };
 
-struct rpcif_impl xspi_impl = {
+static const struct rpcif_impl xspi_impl = {
 	.hw_init = xspi_hw_init_impl,
 	.prepare = xspi_prepare_impl,
 	.manual_xfer = xspi_manual_xfer_impl,