dt-bindings: mmc: tegra: Document Tegra186 compatible string

The SDHCI controller found on Tegra186 in very similar to the controller
found on earlier generations of Tegra.
Signed-off-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/nvidia,tegra20-sdhci.txt

@@ -13,6 +13,7 @@ Required properties:
   - "nvidia,tegra114-sdhci": for Tegra114
   - "nvidia,tegra124-sdhci": for Tegra124 and Tegra132
   - "nvidia,tegra210-sdhci": for Tegra210
+  - "nvidia,tegra186-sdhci": for Tegra186
 - clocks : Must contain one entry, for the module clock.
   See ../clocks/clock-bindings.txt for details.
 - resets : Must contain an entry for each entry in reset-names.