scripts: dts: Consistently open text files with utf-8.

Zephyr codebase standardizes in UTF-8 as file encoding. To
accommodate this, we explicitly pass encoding="utf-8" to Python's
open() function, to be independent of any locale setting of a
particular system (e.g., CI/build systems oftentimes have "C",
i.e. ASCII-only, locale). In a few places, we lacked this parameter,
so add it consistently.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>