Documentation: mmc: sdhci-of-arasan: Add soc-ctl-syscon for corecfg regs

Documentation: mmc: sdhci-of-arasan: Add soc-ctl-syscon for corecfg regs

As can be seen in Arasan's datasheet [1] there are several "corecfg"
settings in their SDHCI IP Block that are supposed to be controlled by
software.  Although the datasheet referenced is a bit vague about how to
access corecfg, in Figure 5 you can see that for Arasan's PHY (a
separate component than their SDHCI component) they describe the
"phyctrl" registers as being "FROM SOC CTL REG", implying that it's up
to the licensee of the Arasan IP block to implement these registers.  It
seems sane to assume that the "corecfg" registers in their SDHCI IP
block works in a similar way for all licensees of the IP Block.

Device tree has a model that allows a device to get a reference to
random registers located elsewhere in the SoC: sysctl.  Let's leverage
this model and allow adding a sysctl reference to access the control
registers for the Arasan SDHCI PHYs.

Having a reference to the control registers doesn't do much for us on
its own since the Arasan spec doesn't specify how these corecfg values
are laid out in memory.  In the SDHCI driver we'll need a map detailing
where each corecfg can be found in each implementation.  This map can be
found using the primary compatible string of the SDHCI device.  In that
spirit, document that existing rk3399 device trees already have a
specific compatible string, though up to now they've always been relying
on the driver supporting the generic.

Note that since existing devices seem to work fairly well as-is, we'll
list the syscon reference as "optional", but it's likely that we'll run
into much fewer problems if we can actually set the proper values in the
syscon, so it is strongly suggested that any SoCs where we have a map to
set the corecfg also include a reference to the syscon.

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

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>