Added possibility to use +8 dBm TX power for micro:bit v2 that supported by nRF52833

[BooBSD]

Hello.
According to nRF52833 specs, SoC is supported TX power up to +8 dBm. But now we can use only +4 dBm maximum from micropython.
This PR adds possibility to use +8 dBm TX power for micro:bit v2.

How to use:

import radio

radio.on()
radio.config(power=8)
radio.send('+8 dBm!')

Thank you.