Documentation: dt: Add binding for /secure-chosen/stdout-path

Some platforms may use a single device tree to describe two address
spaces, as described in d9f43babb998 ("Documentation: dt: Add bindings
for Secure-only devices"). For these platforms it makes sense to define
a secure counterpart of /chosen, namely: /secure-chosen. This new node
is meant to be used by the secure firmware to pass data to the secure
OS. Only the stdout-path property is supported for now.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/secure.txt

@@ -32,7 +32,8 @@ describe the view of Secure world using the standard bindings. These
 secure- bindings only need to be used where both the Secure and Normal
 world views need to be described in a single device tree.
 
-Valid Secure world properties:
+Valid Secure world properties
+-----------------------------
 
 - secure-status : specifies whether the device is present and usable
   in the secure world. The combination of this with "status" allows
@@ -51,3 +52,19 @@ Valid Secure world properties:
    status = "disabled"; secure-status = "okay";     /* S-only */
    status = "disabled";                             /* disabled in both */
    status = "disabled"; secure-status = "disabled"; /* disabled in both */
+
+The secure-chosen node
+----------------------
+
+Similar to the /chosen node which serves as a place for passing data
+between firmware and the operating system, the /secure-chosen node may
+be used to pass data to the Secure OS. Only the properties defined
+below may appear in the /secure-chosen node.
+
+- stdout-path : specifies the device to be used by the Secure OS for
+  its console output. The syntax is the same as for /chosen/stdout-path.
+  If the /secure-chosen node exists but the stdout-path property is not
+  present, the Secure OS should not perform any console output. If
+  /secure-chosen does not exist, the Secure OS should use the value of
+  /chosen/stdout-path instead (that is, use the same device as the
+  Normal world OS).