pseq: Document enhanced JSON config file format

The JSON config file format is being enhanced for the
phosphor-power-sequencer application.

The enhancements are intended to provide the following be

pseq: Document enhanced JSON config file format

The JSON config file format is being enhanced for the
phosphor-power-sequencer application.

The enhancements are intended to provide the following benefits:
* Generalize the format so it can be used for any power sequencer
device. The current format is intended only for UCD90XXX devices.
* Remove the "pins" array. Move the GPIO pin information to an optional
property of a rail object. This will eliminate some duplication and
make pgood isolation more flexible.
* Change the meaning of the rail order within the array of rails.
* In the current format, the array index implies the PMBus PAGE. This
causes problems because it is impossible to ignore a PAGE that is
not a valid voltage rail. It also does not provide information
on the power on sequence, which is often different than the PAGE
order.
* In the new format, the rails in the array will be in power on
sequence order. The PMBus page will be explicitly defined using
a new property.
* Add rail property that specifies the PMBus PAGE.
* Add rail property that specifies the pgood status should be obtained
using the PMBus STATUS_VOUT command.
* Add rail property that specifies the pgood status should be obtained
by comparing the output voltage (READ_VOUT) to the undervoltage
(VOUT_UV_FAULT_LIMIT) and overvoltage (VOUT_OV_FAULT_LIMIT) limits.
* Add rail property that specifies the pgood status should be obtained
from a GPIO.

Note: This commit contains only the new JSON file format documentation.
Subsequent commits will contain the new C++ implementation and new
versions of the existing JSON files. The current documentation, C++,
and JSON files will be retained and will remain active until all of the
new support has been merged.

Change-Id: I358a885f1675a9dd8f9ae0115e560abb3c0828fe
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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