boards: st: remove U suffix from "clocks" in DTS

PR 79683 added unnecessary U suffix to constants in DTS.
This bloats the files for no reason and is starting to spread as people
use the DTS for reference in other places, and so copy this bad pattern.

Remove the useless U suffixes in DTS files for ST boards.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>