hw/arm/virt: add DT property /secure-chosen/stdout-path indicating secure UART

hw/arm/virt: add DT property /secure-chosen/stdout-path indicating secure UART

Bindings for /secure-chosen and /secure-chosen/stdout-path have been
proposed 1.5 years ago [1] and implemented in OP-TEE at the same time [2].
They've now been officially agreed on, so we can implement them
in QEMU.

This patch creates the property when the machine is secure.

[1] https://patchwork.kernel.org/patch/9602401/
[2] https://github.com/OP-TEE/optee_os/commit/4dc31c52544a

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Message-id: 20181005080729.6480-1-jerome.forissier@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: commit message tweak]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>