add PocketBeagle example to blink LED

add PocketBeagle example to blink LED

Note: Python programs using Blinka on PocketBeagle and BeagleBone need to use `sudo`.  Refer to this issue for more information:
https://github.com/adafruit/Adafruit_Python_PlatformDetect/issues/20

Here is the output of this program running OK on a PocketBeagle:
debian@beaglebone:~/Adafruit_Blinka/examples$ sudo python3 pb_digitalio.py
[sudo] password for debian:
hello blinky!