Add default devices and tweak brightness in boards/pewpew10

[deshipu]

• add default pins for i2c, spi and uart
• add default i2c, spi and uart devices
• ignore usb pins
• tweak brightness and refresh rate of the display matrix

[jerryneedell]

Downloaded and tested on pewpew10 --- still functions normally -- I did not do any specific tests.
Imported board to verify pin definitions are seen.

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Adafruit CircuitPython 4.0.0-beta.3-5-gcaf51cf4e on 2019-03-11; PewPew 10.2 with samd21e18
>>> import board
>>> dir(board)
['DAC', 'I2C', 'MISO', 'MOSI', 'P1', 'P2', 'P3', 'P4', 'P5', 'P6', 'P7', 'RX', 'SCK', 'SCL', 'SDA', 'SPI', 'TX', 'UART', '_BUTTONS', '_C1', '_C2', '_C3', '_C4', '_C5', '_C6', '_C7', '_C8', '_R1', '_R2', '_R3', '_R4', '_R5', '_R6', '_R7', '_R8']
>>> 

Looks like it "does no harm" ;-)

[tannewt]

Looks good! Thank you!