[Hardware] Support the nRF52840-DK

[jamesmunns]

The Nordic nRF52840-DK is a relatively popular dev board, using the same main CPU as the currently supported Feather Express board.

It also has been used in a number of written guides for Rust, including Ferrous Systems' trainings and Knurling sessions, which means many Rust folks will already have one in a drawer.

In order to support this board, two main things need to happen:

• We need some way of handling multiple pin mappings, to handle the difference between the two boards (either statically through cfg or at runtime)
• We need some way of handling multiple QSPI parts, to handle the different flash memory available between the two boards (again statically through cfg or at runtime)

This would be a great first issue for someone familiar with embedded Rust, and interested in getting started with the kernel!