pseq: Create GPIO class hierarchy

Create GPIO class hierarchy within the phosphor-power-sequencer
application.

The GPIO abstract base class declares virtual methods for requesting,
reading, writing

pseq: Create GPIO class hierarchy

Create GPIO class hierarchy within the phosphor-power-sequencer
application.

The GPIO abstract base class declares virtual methods for requesting,
reading, writing, and releasing a GPIO.

The BMCGPIO subclass provides a real implementation using the libgpiod
API.

The MockGPIO subclass provides a mock implementation for automated
testing.

Tested:
* Verified GPIO could be read
* Verified GPIO could be written
* Verified error paths
* Verified MockGPIO could be used in an automated test

Change-Id: Ie5808988c5d743c44735f5bc4c09cd15b397518b
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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