mmc: sdhci_am654: Enable tuning for SDR50

According to the SW tuning App note[1], tuning is required for all
UHS speed modes. Tuning for SDR50 is not enabled in Capabilities by
default so enable it from the CTL_CFG registers.

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



Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Link: https://lore.kernel.org/r/20200923105206.7988-7-faiz_abbas@ti.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>