doc: scripts: gen_devicetree_rest: use utf-8 encoding

[gmarull]

Use utf-8 encoding when opening vendor prefix files. Note that utf-8 is enforced in other scripts, e.g. gen_kconfig_rest, gen_helpers, etc.

Fixes #34076

Alternative to #35234

[mbolivar-nordic]

There was another issue related to breathe mentioned in #34076 which I think neither this nor #35234 addresses:

#34076 (comment)

Any idea where that I/O error is happening?

[gmarull]

There was another issue related to breathe mentioned in #34076 which I think neither this nor #35234 addresses:

#34076 (comment)

Any idea where that I/O error is happening?

breathe is responsible of handling Doxygen XML files. Since Doxygen seems to process input in UTF-8, I imagine it's breathe not reading XML files using UTF-8 on Windows. Some suspicious stuff: https://github.com/michaeljones/breathe/search?q=ascii