Raspberry Pi: Keep warnings from firmware in DT, if any

Raspberry Pi: Keep warnings from firmware in DT, if any

The /chosen/user-warnings property is created by the RPi firmware if
there are warnings to report, keep it to make debugging easier.

For example, if the firmware config.txt contains
"dtoverlay=error-example" and that example references an undefined
symbol "&nosuchdev" the warning can be read after boot:

$ cat /proc/device-tree/chosen/user-warnings
dterror: can't find symbol 'nosuchdev'
Failed to resolve overlay 'error-example'

Signed-off-by: Fiona Klute <fiona.klute@gmx.de>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>