mmc: sh_mobile_sdhi: only change the clock on RCar Gen2+

We had a regression on r8a7740 where the SDHI clock was a generic
peripheral clock, so changing its rate was not desired. This should be
fixed in the clock driver. However, it also shows that the new clock
calculation should only be used on tested systems. Add a check for that.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>