bindings: pinctrl: stm32: add binding for STM32N6 series pinctrl

Add a new binding for the pinctrl controller of STM32N6 series.

The specificity of this series is the "I/O retime" feature not present on
other series. This new binding exposes pinctrl properties to configure this
feature.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>