pseq: Move common code to BasicDevice class

Create a new sub-class of PowerSequencerDevice called BasicDevice that
implements basic functionality.

BasicDevice implements the following:
- Data membe

pseq: Move common code to BasicDevice class

Create a new sub-class of PowerSequencerDevice called BasicDevice that
implements basic functionality.

BasicDevice implements the following:
- Data members and methods for the power sequencer properties from the
JSON configuration file.
- Methods that utilize the named GPIOs, such as powerOn() and
getPowerGood().

This functionality was previous duplicated in both StandardDevice and
GPIOsOnlyDevice. Change both of those classes to inherit from
BasicDevice.

Tested:
* Ran automated tests

Change-Id: Id80e34dca0d2412ae10abe9962eca632d1909962
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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