Add support for Radxa Rock 3B

[fb0u]

Before modifications:

>>> import adafruit_platformdetect
>>> detector = adafruit_platformdetect.Detector()
>>> detector.board.id
>>> detector.chip.id
'RK3568'
>>> detector.get_device_model()
'Radxa ROCK 3 Model B\x00'

After modifications:

>>> import adafruit_platformdetect
>>> detector = adafruit_platformdetect.Detector()
>>> detector.board.id
'ROCK_3B'

[makermelissa]

Looks good to me.