memory: renesas-rpc-if: avoid use of undocumented bits

memory: renesas-rpc-if: avoid use of undocumented bits

Instead of writing fixed values with undocumented bits which happen to
be set on some SoCs, better switch to read-modify-write operations
changing only bits which are documented. This is way more future-proof
as we don't know yet how these bits may be on upcoming SoCs.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20211117093710.14430-1-wsa+renesas@sang-engineering.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>