rk3399: mmc: hs400 strobe clock > 200Mhz

[Chris Ruehl]

Hi

I reach out with a problem we identify with SanDisk 8G eMMC
85% of our production works well with hs400es and hs400 configuration,
the other 15% have problems detecting the chip.
For the failure analysis for the unstable boards have the strobe
clock of more then 200Mhz. We measured 204.7Mhz, looks like a
chip related problem.

Anything we can improve on the rockchip / mmc stack?

-Chris