dts: arm: st: stm32l1: remove U suffix from "clocks" in DTSI

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

Remove the useless U suffixes in DTSI files for this series.

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