dt-bindings: mmc: Add DQS trim value to Tegra SDHCI

Document HS400 DQS trim value device tree property.

Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>