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>
diff --git a/board/raspberrypi/rpi/rpi.c b/board/raspberrypi/rpi/rpi.c
index fe6153b..43efb0d 100644
--- a/board/raspberrypi/rpi/rpi.c
+++ b/board/raspberrypi/rpi/rpi.c
@@ -594,6 +594,9 @@
 	/* kernel address randomisation seed as provided by the firmware */
 	copy_property(fdt, fw_fdt, "/chosen", "kaslr-seed");
 
+	/* warnings from the firmware (if any) */
+	copy_property(fdt, fw_fdt, "/chosen", "user-warnings");
+
 	/* address of the PHY device as provided by the firmware  */
 	copy_property(fdt, fw_fdt, "ethernet0/mdio@e14/ethernet-phy@1", "reg");
 }