mmc: sdhci-of-arasan: Add ability to export card clock

Some SD/eMMC PHYs (like the PHY from Arasan that is designed to work
with arasan,sdhci-5.1) need to know the card clock in order to function
properly.  Let's add the ability to expose this clock.  Any PHY that
needs to know the clock rate can add a reference and query the clock
rate.

At the moment we register a CLK_GET_RATE_NOCACHE clock that simply
allows querying the clock.  This allows us to be less intrusive with
regards to the main SDHCI driver, which has complex logic for adjusting
the SD clock.  Right now we always fully power cycle the PHY when the
clock changes and that gives the PHY a good chance to query our clock.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>