pseq: Add GPIO methods to PowerSequencerDevice

Add the following methods to the PowerSequencerDevice class hierarchy:
* getPowerControlGPIO()
* getPowerGoodGPIO()
* powerOn()
* powerOff()
* getPower

pseq: Add GPIO methods to PowerSequencerDevice

Add the following methods to the PowerSequencerDevice class hierarchy:
* getPowerControlGPIO()
* getPowerGoodGPIO()
* powerOn()
* powerOff()
* getPowerGood()

Create GPIO objects in the PowerSequencerDevice sub-classes to implement
these new methods using named GPIOs.

Update the BMCGPIO class to specify an initial value when requesting the
GPIO line for write.

Tested:
* Ran automated test cases
* Tested the following GPIOsOnlyDevice methods in simulation
* powerOn()
* powerOff()
* getPowerGood()
* Tested the following StandardDevice methods in simulation
* powerOn()
* powerOff()
* getPowerGood()
* Tested the following BMCGPIO methods in simulation
* BMCGPIO()
* requestRead()
* requestWrite()
* getValue()
* setValue()
* release()
* ~BMCGPIO()

Change-Id: Icaeb643e4da1c36786714fc58a174f53fa7c4501
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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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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