sdhci: Add generic HS200 tuning and clock fixes

[manu]

Summary

Allow HS200 tuning only above 100MHz timings, let the host driver keep a
fixed maximum clock, and stop clearing max_clk in the generic FDT attach so
that a controller driver can set it before attach.

[mmel]

Unfortunately, I know very little about MMC, so I'm not the right person to review this.