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>

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

@@ -67,6 +67,10 @@ Optional properties for Tegra210 and Tegra186:
 - nvidia,pad-autocal-pull-up-offset-hs400,
   nvidia,pad-autocal-pull-down-offset-hs400 : Specify drive strength
   calibration offsets for HS400 mode.
+- nvidia,default-tap : Specify the default inbound sampling clock
+  trimmer value for non-tunable modes.
+- nvidia,default-trim : Specify the default outbound clock trimmer
+  value.
 
   Notes on the pad calibration pull up and pulldown offset values:
     - The property values are drive codes which are programmed into the
@@ -77,6 +81,13 @@ Optional properties for Tegra210 and Tegra186:
     - The SDR104 and HS400 timing specific values are used in
       corresponding modes if specified.
 
+  Notes on tap and trim values:
+    - The values are used for compensating trace length differences
+      by adjusting the sampling point.
+    - The values are programmed to the Vendor Clock Control Register.
+      Please refer to the reference manual of the SoC for correct
+      values.
+
 Example:
 sdhci@700b0000 {
 	compatible = "nvidia,tegra210-sdhci", "nvidia,tegra124-sdhci";