patman: encode CC list to UTF-8

This change encodes the CC list to UTF-8 to avoid failures on
maintainer-addresses that include non-ASCII characters (observed on
Debian 7.11 with Python 2.7.3).

Without this, I get the following failure:
  Traceback (most recent call last):
    File "tools/patman/patman", line 159, in <module>
      options.add_maintainers)
    File "[snip]/u-boot/tools/patman/series.py", line 234, in MakeCcFile
      print(commit.patch, ', '.join(set(list)), file=fd)
  UnicodeEncodeError: 'ascii' codec can't encode character u'\xfc' in position 81: ordinal not in range(128)
from Heiko's email address:
  [..., u'"Heiko St\xfcbner" <heiko@sntech.de>', ...]

While with this change added this encodes to:
  "=?UTF-8?q?Heiko=20St=C3=BCbner?= <heiko@sntech.de>"

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Tested-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>