ARM: dts: stm32: update pwm-cells for LPTimer on stm32h743

LPTimer pwm cells should be updated to 3, to allow initialization of
channel, period and polarity.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>