memory: renesas-rpc-if: Add RZ/G3E xSPI support

Add support for RZ/G3E xSPI. Compared to RPC-IF, it can support writes on
memory-mapped area. Introduce struct rpcif_impl for holding the function
pointers and data to handle the differences between xspi and rpc-if
interface.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20250424090000.136804-7-biju.das.jz@bp.renesas.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

Author: bijudas