mmc: sdhci-of-arasan: Don't power PHY w/ slow/no clock

PHY intended to be used with the Arasan SDHCI 5.1 controller has trouble
turning on when the card clock is slow or off.  Strangely these problems
appear to show up consistently on some boards while other boards work
fine, but on the boards where it shows up the problem reproduces 100% of
the time and is quite consistent in its behavior.

These problems can be fixed by always making sure that we power on the
PHY (and turn on its DLL) when the card clock is faster than about 50
MHz.  Once on, we need to make sure that we never power down the PHY /
turn off its DLL until the clock is faster again.

We'll add logic for handling this into the sdhci-of-arasan driver.  Note
that right now the only user of a PHY in the sdhci-of-arasan driver is
arasan,sdhci-5.1.  It's presumed that all arasan,sdhci-5.1 PHY
implementations need this workaround, so the logic is only contingent on
having a PHY to control.  If future Arasan controllers don't have this
problem we can add code to decide if we want this flow or not.

Also note that we check for slow clocks by checking for <= 400 kHz
rather than checking for 50 MHz.  This keeps things the most consistent
and also means we can power the PHY on at max speed (where the DLL will
lock fastest).  Presumably anyone who intends to run with a card clock
of < 50 MHz and > 400 kHz will be running on a device where this problem
is fixed anyway.

I believe this brings some resolution to the problems reported before.
See the commit 6fc09244d74d ("mmc: sdhci-of-arasan: Revert: Always power
the PHY off/on when clock changes").

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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