MAINTAINERS: Add drivers/firmware/google/ entry

These are mostly used for Chrome platforms, so group it in with the same
mailing list, repo, and (one) committer.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Acked-by: Julius Werner <jwerner@chromium.org>
Acked-By: Benson Leung <bleung@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20230724081247.678784-1-tzungbi@kernel.org
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>