ARM: dts: Use stdout-path in STM32F429 Discovery board

This patch replaces use of linux,stdout-path by stdout-path as per
"chosen" DT bindings documentation.

Doing that, the "console" argument is no more needed in kernel command
line.

Reported-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>