dts: bindings: stm32 lptimer has no divider on the clock source freq

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 572b2860.

Signed-off-by: Francois Ramu <francois.ramu@st.com>