add more plumbing for SPI on BeagleBone

reuse the Raspberry Pi SPI class as both are
Linux boards using spidev.  The main difference
is the pins numbers are different.

Testing with Adafruit_CircuitPython_BME280 in SPI mode.

TODO: wire up the board to the BeagleBone Black

Current output:

debian@beaglebone:~/Adafruit_CircuitPython_BME280$ sudo python3 examples/bme280_simpletest.py
SPI(): __init()
SPI(): beaglebone_black: from adafruit_blinka.microcontroller.raspi_23.spi import SPI as _SPI
spiPorts: ((0, P9_22, P9_18, P9_21), (1, P9_22, P9_18, P9_21))
for:
0 P9_22 P9_18 P9_21
Line 91
<class 'adafruit_blinka.microcontroller.raspi_23.spi.SPI'>
<adafruit_blinka.microcontroller.raspi_23.spi.SPI object at 0xb6b3c890>
Traceback (most recent call last):
  File "examples/bme280_simpletest.py", line 15, in <module>
    bme280 = adafruit_bme280.Adafruit_BME280_SPI(spi, bme_cs)
  File "/usr/local/lib/python3.5/dist-packages/adafruit_circuitpython_bme280-2.1.1.dev3+g1416d93-py3.5.egg/adafruit_bme280.py", line 247, in __init__
  File "/usr/local/lib/python3.5/dist-packages/adafruit_circuitpython_bme280-2.1.1.dev3+g1416d93-py3.5.egg/adafruit_bme280.py", line 78, in __init__
RuntimeError: Failed to find BME280! Chip ID 0x0