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 d9f43bab

 ("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>