ARM: dts: stm32: use dedicated files for pinctrl on stm32f7 family

Currently, same stm32f746-pinctrl driver is used for stm32f746 and
stm32f769 MCU. As pin map is different between those 2 MCUs,
a stm32f769-pinctrl driver has been recently added.
This patch
 -allows to use stm32f769-pinctrl driver for stm32f769 boards
 -reworks stm32 devicetree files to fit with stm32f746 / stm32f769

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>