pylibfdt: Fix "invalid escape sequence '\w'" in setup.py

Once u-boot's build system invokes

python3 scripts/dtc/pylibfdt/setup.py --quiet build_ext --inplace

it may fail with

scripts/dtc/pylibfdt/setup.py:40: SyntaxWarning: invalid escape sequence '\w'
  RE_KEY_VALUE = re.compile('(?P<key>\w+) *(?P<plus>[+])?= *(?P<value>.*)$')

depending on the used Python version.

Explicitly mark the regex string as raw string to avoid the warning.

Signed-off-by: Florian Schmaus <flo@geekplace.eu>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>