Support NRF52840_DK in Mbed CE

[multiplemonomials]

Summary of changes

This PR adds Mbed CE support for this target. This includes:

• Creating an upload method config file
• Updating the linker script to use memory banks
• Updating PinNames.h to the new style (using FF_ARDUINO_UNO and #define'd LEDs and buttons)

Impact of changes

• Proper Mbed CE support for NRF2840_DK
• Bug fixed where passing MBED_UPLOAD_SERIAL_NUMBER didn't work for flashing with J-Link
• Bug fixed where the MBED usb upload method would ignore devices with a matching serial number if it couldn't detect the name

Migration actions required

Documentation

I updated the doc site to explain how to configure USB TTYs on this target (which took me a bit of digging!)

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] 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)
[X] Tests / results supplied as part of this PR

Test run completed: https://mbed-ce.github.io/mbed-ce-test-tools/tests/