Revert "dts: bindings: LPtimer of stm32 has a x2 factor on its clock"
The stm32u5 lptim clock source has no prescaler to divide the
the LPTIM input clock frequency : no property required.
This reverts commit 572b286010.
Signed-off-by: Francois Ramu <francois.ramu@st.com>