scripts: dts: properly escape string properties

Fixed escaping of double quotes, backslashes, and new line characters
so they can be used in string properties.

Previously, double quotes and backslashes were escaped in gen_defines.py
but not in gen_dts_cmake.py, and new lines were not escaped in either,
so using any of these characters would break the build.

Signed-off-by: Joel Spadin <joelspadin@gmail.com>