dt-bindings: mmc: renesas,sdhi: Add optional SDnH clock

This only applies to R-Car Gen2 and later generations, so we need to
distinguish.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20211115160600.4455-1-wsa+renesas@sang-engineering.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>