mmc: sdhci-of-arasan: Properly set corecfg_baseclkfreq on rk3399

In the the earlier change in this series ("Documentation: mmc:
sdhci-of-arasan: Add soc-ctl-syscon for corecfg regs") we can see the
mechansim for specifying a syscon to properly set corecfg registers in
sdhci-of-arasan.  Now let's use this mechanism to properly set
corecfg_baseclkfreq on rk3399.

>From [1] the corecfg_baseclkfreq is supposed to be set to:
  Base Clock Frequency for SD Clock.
  This is the frequency of the xin_clk.

This is a relatively easy thing to do.  Note that we assume that xin_clk
is not dynamic and we can check the clock at probe time.  If any real
devices have a dynamic xin_clk future patches could register for
notifiers for the clock.

At the moment, setting corecfg_baseclkfreq is only supported for rk3399
since we need a specific map for each implementation.  The code is
written in a generic way that should make this easy to extend to other
SoCs.  Note that a specific compatible string for rk3399 is already in
use and so we add that to the table to match rk3399.

[1]: https://arasan.com/wp-content/media/eMMC-5-1-Total-Solution_Rev-1-3.pdf



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