mmc: sdhci_am654: Add support for software tuning

mmc: sdhci_am654: Add support for software tuning

With the new SW tuning App note[1], a custom tuning algorithm is
required for eMMC HS200, HS400 and SD card UHS modes. The algorithm
involves running through the 32 possible input tap delay values and
sending the appropriate tuning command (CMD19/21) for each of them
to get a fail or pass result for each of the values. Typically, the
range will have a small contiguous failing window. Considering the
tuning range as a circular buffer, the algorithm then sets a final
tuned value directly opposite to the failing window.

[1] https://www.ti.com/lit/pdf/spract9

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>
Link: https://lore.kernel.org/r/20200923105206.7988-6-faiz_abbas@ti.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>