dt-bindings: mmc: renesas,sdhi: Document R-Car S4-8 and generic Gen4 support

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20220603233810.21972-1-wsa+renesas@sang-engineering.com


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