boards: Add BBC MicroBit V2 support

The BBC micro:bit v2 is a mini-computer that has been
designed to make the coding fun and easy to learn.

The micro:bit v2 is completely programmable so you can
easily bring your ideas to life! From making games to
creating music and even controlling robots.

The micro:bit comes with neat hardware such as a 25 LED
display, buttons, in-built speakers, Bluetooth 5 & Mesh
connectivity and sensors for temperature, motion & light.

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>