dt-bindings: mmc: Add Tegra SDHCI sampling trimmer values

Document the Tegra SDHCI inbound and outbound sampling trimmer values.

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>