soc: arm: Device tree refactor and support for stm32f765xx

Created a seperate device tree file for the stm32f765.
Moved common nodes from the stm32f767 device tree file to the new file and
based the stm32f767 off the stm32f765.

Signed-off-by: Rahul Arasikere <arasikere.rahul@gmail.com>