Add P2_25 pin to fix error on PocketBeagle

Add P2_25 pin to fix error on PocketBeagle

Add P2_25 pin to fix error on PocketBeagle.  This is the error that occurs without this fix:
```
Traceback (most recent call last):
  File "examples/bbb_digitalio.py", line 10, in <module>
    import board
  File "/usr/local/lib/python3.5/dist-packages/Adafruit_Blinka-1.2.9.dev5+gcbfa2eb.d20190330-py3.5.egg/board.py", line 59, in <module>
    from adafruit_blinka.board.beaglebone_pocketbeagle import *
  File "/usr/local/lib/python3.5/dist-packages/Adafruit_Blinka-1.2.9.dev5+gcbfa2eb.d20190330-py3.5.egg/adafruit_blinka/board/beaglebone_pocketbeagle.py", line 114, in <module>
    MOSI_1 = pin.P2_25
AttributeError: module 'adafruit_blinka.microcontroller.am335x.pin' has no attribute 'P2_25'
```
Refer to comments in PR #101