scripts: dts: devicetree.py: Fix pylint warning for iffy \w escape

"\w" gives a two-character string, but is iffy, because it relies on \w
not being defined as an escape sequence. r"\w" is better.

Fixes this pylint warning:

    scripts/dts/devicetree.py:134:0: W1401: Anomalous backslash in
    string: '\w'. String constant might be missing an r prefix.
    (anomalous-backslash-in-string)

Wondering if I should exclude the old DTS scripts from the pylint CI
check, but doesn't hurt to fix it at least.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>