Fix syntax highlighting

Github syntax highlighting broke at some point over the past month or
so. From its documentation, it is not clear to me what changeds. This is
somewhat of a longshot attempt to making it work again.

https://github.com/github-linguist/linguist/blob/master/docs/overrides.md

.gitattributes

@@ -1,2 +1,3 @@
-*.dtsi linguist-language=C++
-*.keymap linguist-language=C++
+# Highlight keymap files as C++, which is better than nothing.
+*.dtsi text linguist-language=C++
+*.keymap text linguist-language=C++