fdt_support: refactor fdt_fixup_stdout() function

- Do not use a deep indentation. We have only 80-character
   on each line and 1 indentation consumes 8 spaces. Before the
   code moves far to the right, you should consider to
   fix your code. See Linux Documentation/CodingStyle.

 - Add CONFIG_OF_STDOUT_VIA_ALIAS and OF_STDOUT_PATH macros
   only to their definition. Do not add them to both
   callee and caller. This is a tip to avoid using #ifdef
   everywhere.

 - OF_STDOUT_PATH and CONFIG_OF_STDOUT_VIA_ALIAS are exclusive.
   If both are defined, the former takes precedence.
   Do not try to fix-up "linux,stdout-path" property twice.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>