[draft] Add Arduino Giga support

[multiplemonomials]

Summary of changes

This PR updates Mbed OS to support the Arduino Giga. It is still in progress. So far:
[X] Port over Arduino's patches to Mbed CE
[X] Disentangle the horrible, awful spaghetti code Arduino created for the wifi driver into something vaguely workable
[X] Get wifi driver actually running
[X] Get QSPI flash operational
[ ] Add support for using the board with the bootloader instead of via an ST-Link
[ ] Get network tests fully working for wifi boards
[ ] Debug why a thread sleep is needed in the wifi driver in order to connect to networks
[ ] Test bluetooth

Impact of changes

Migration actions required

Documentation

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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