ARM: DT: tegra: Add board level compatible properties

The compatible properties of Tegra SoC based boards or machines need
to be documented. This patch adds these board levle compatible
properties into device tree binding document.
Signed-off-by: NBryan Wu <pengw@nvidia.com>
Signed-off-by: NStephen Warren <swarren@nvidia.com>

Documentation/devicetree/bindings/arm/tegra.txt

 NVIDIA Tegra device tree bindings
 -------------------------------------------
 
-Boards with the tegra20 SoC shall have the following properties:
+SoCs
+-------------------------------------------
 
-Required root node property:
+Each device tree must specify which Tegra SoC it uses, using one of the
+following compatible values:
 
-compatible = "nvidia,tegra20";
+  nvidia,tegra20
+  nvidia,tegra30
 
-Boards with the tegra30 SoC shall have the following properties:
+Boards
+-------------------------------------------
 
-Required root node property:
+Each device tree must specify which one or more of the following
+board-specific compatible values:
 
-compatible = "nvidia,tegra30";
+  ad,medcom-wide
+  ad,plutux
+  ad,tamonten
+  ad,tec
+  compal,paz00
+  compulab,trimslice
+  nvidia,cardhu
+  nvidia,cardhu-a02
+  nvidia,cardhu-a04
+  nvidia,harmony
+  nvidia,seaboard
+  nvidia,ventana
+  nvidia,whistler