dt-bindings: pwm: stm32-lp: Document pin control sleep state

Add documentation for pin control sleep state on STM32 LPTimer PWM.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>