pseq: Update internal design document

Update the internal design documentation for the
phosphor-power-sequencer application:
* Add new System and Chassis classes
* Remove DeviceFinder class

Change-Id: I5865d00d8ad98d43053788d3ec7f7c489903ec14
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
diff --git a/phosphor-power-sequencer/docs/internal_design.md b/phosphor-power-sequencer/docs/internal_design.md
index 750b0cd..82b7ecd 100644
--- a/phosphor-power-sequencer/docs/internal_design.md
+++ b/phosphor-power-sequencer/docs/internal_design.md
@@ -11,18 +11,19 @@
   - Created in `main()`. Handles the event loop.
   - Sub-class of PowerInterface that provides a concrete implementation of the
     `org.openbmc.control.Power` D-Bus interface.
-  - Finds and loads the JSON configuration file.
-  - Finds power sequencer device information.
-  - Creates a sub-class of PowerSequencerDevice that matches power sequencer
-    device information.
+  - Finds and loads the JSON configuration file. This creates an instance of the
+    System class.
   - Powers the chassis on and off using the `power-chassis-control` named GPIO.
   - Monitors the chassis pgood status every 3 seconds using the
     `power-chassis-good` named GPIO.
   - Enforces a minimum power off time of 15 seconds from cold start and 25
     seconds from power off.
-- DeviceFinder
-  - Finds power sequencer device information on D-Bus published by
-    EntityManager.
+- System
+  - The computer system being controlled and monitored by the BMC.
+- Chassis
+  - A chassis within the system. Chassis are typically a physical enclosure that
+    contains system components such as CPUs, fans, power supplies, and PCIe
+    cards.
 - Rail
   - A voltage rail that is enabled or monitored by the power sequencer device.
 - PowerSequencerDevice