mmc: renesas_sdhi: add R-Car Gen4 fallback compatibility string

For now, Gen4 is treated the same as Gen3. But we still want a seperate
fallback just in case.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20220603233300.21789-1-wsa+renesas@sang-engineering.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>