mmc: tegra: Disable card clock during tuning cmd on Tegra210

Implement tegra210_sdhci_writew() to disable card clock and issue a
reset when the tuning command is sent. This is done to prevent an
intermittent hang with around 10 % failure rate during tuning.

Add tegra186_sdhci_ops because this workaround is specific to Tegra210.

Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>