pseq: Create PMBusDriverDevice class

Create the PMBusDriverDevice class in the phosphor-power-sequencer
application.

This class is used for power sequencer devices that are bound to a PMBus
device

pseq: Create PMBusDriverDevice class

Create the PMBusDriverDevice class in the phosphor-power-sequencer
application.

This class is used for power sequencer devices that are bound to a PMBus
device driver. It obtains PMBus information from sysfs/hwmon files
created by the driver.

Tested:
* Performed all of the following tests on Rainier and Everest systems
* Correct sysfs/hwmon files and directories found
* GPIO values obtained successfully
* STATUS_WORD values obtained successfully
* STATUS_VOUT values obtained successfully
* READ_VOUT values obtained successfully
* VOUT_UV_FAULT_LIMIT values obtained successfully
* Mapping built from PMBus PAGE numbers to hwmon file numbers
* Verified error paths and exceptions thrown

Change-Id: I2efd3146fa08d3584857c94c2bbbf691b1e2ad7d
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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