pseq: Add getRails() method to device hierarchy

Add a method to get the voltage rails that are being enabled/monitored
by a power sequencer device.

Tested:
* Verified all gtests ran successfully

Change-Id: I04b3e9b0854c110858daf30dbf35dc44ab4ade0e
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
diff --git a/phosphor-power-sequencer/src/rail.hpp b/phosphor-power-sequencer/src/rail.hpp
index 5a49e13..b4b6f95 100644
--- a/phosphor-power-sequencer/src/rail.hpp
+++ b/phosphor-power-sequencer/src/rail.hpp
@@ -15,7 +15,6 @@
  */
 #pragma once
 
-#include "power_sequencer_device.hpp"
 #include "services.hpp"
 
 #include <cstdint>
@@ -28,6 +27,9 @@
 namespace phosphor::power::sequencer
 {
 
+// Forward declarations to avoid circular dependencies
+class PowerSequencerDevice;
+
 /**
  * @struct GPIO
  *