dt-bindings: pwm-stm32: Document pinctrl sleep state

Add documentation for pinctrl sleep state that can be used by STM32
timers PWM.

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