dts: arm: st: u0: disable lptim2 by default

LPTIM2 is enabled by default; disable it to avoid
the build failure:
"Error: Only one LPTIM instance should be enabled"
when LPTIM1 is also enabled in the DTS.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>