treewide: replace 'linux, stdout-path' with 'stdout-path'

This follows a similar upstream patch by Rob Herring:

|commit 78e5dfea84dc15d69940831b3981b3014d17222e
|Author: Rob Herring <robh@kernel.org>
|Date:   Wed Feb 28 16:44:06 2018 -0600
|    powerpc: dts: replace 'linux,stdout-path' with 'stdout-path'
|
|    'linux,stdout-path' has been deprecated for some time in favor of
|    'stdout-path'. Now dtc will warn on occurrences of 'linux,stdout-path'.
|    Search and replace all the of occurrences with 'stdout-path'.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>