Add Support For Discovery F769I

[sandeepach]

Add mbed Support For Discovery F769I board. I want to try the quad microphones on this device along with Double precision FP.

[0xc0170]

cc @bcostm @LMESTM @adustm

[adustm]

Hello,
@ohagendorf ? Do you still plan to add this board ?
Kind regards

[ohagendorf]

I'm working on it.
And hope to finish it before my holiday.

[ciarmcom]

ARM Internal Ref: IOTMORF-128

[ohagendorf]

please see PR #2504

[sg-]

F769I has been added to the codebase https://github.com/ARMmbed/mbed-os/blob/master/targets/targets.json#L1215