fdt_support: Don't panic if stdout alias is missing

fdt_support: Don't panic if stdout alias is missing

Currently, using fdt_fixup_stdout() on a device tree that is missing
the relevant alias results in this:

WARNING: could not set linux,stdout-path FDT_ERR_NOTFOUND.
ERROR: /chosen node create failed
 - must RESET the board to recover.

FDT creation failed! hanging...### ERROR ### Please RESET the board ###

There is no reason for this to be a fatal error rather than a warning,
and removing this allows for a smooth transition on a platform where
the device tree currently lacks the correct aliases but will have them
in the future.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <yorksun@freescale.com>