STM32F7 series MCUs support

[pic16f887]

I have started with STM32F7 series MCUs support. Please assign this issue to me.

[erwango]

@mjs-arm : you might be interested

[erwango]

@pic16f887, Thanks for doing this work.
As an advice, you could have a look to recent #5949
which was last PR that introduced a new STM32 family to Zephyr.
Your work should be close to that at the end.

[pic16f887]

@erwango, Thank you. I will take it into account.