mmc: renesas_sdhi: Parse DT for SDnH

If there is a SDnH clock provided in DT, let's use it instead of relying
on the fallback.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20211110191610.5664-21-wsa+renesas@sang-engineering.com


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