regulators: Complete PresenceDetection class

Complete implementation of the PresenceDetection class.  Implement the
execute() method that executes one or more actions to determine if a
device is present.

Cache the results so that subsequent calls to execute() will return the
cached value.  Executing the actions may be expensive, requiring one or
more D-Bus calls.

Provide a clearCache() method to be called on each boot to clear the
cached value.

Signed-off-by: Bob King <Bob_King@wistron.com>
Change-Id: Ie0e3dfde10f2df8ca8b56ec14ce723f356e97dfc
diff --git a/phosphor-regulators/src/meson.build b/phosphor-regulators/src/meson.build
index 8238d93..7b852bb 100644
--- a/phosphor-regulators/src/meson.build
+++ b/phosphor-regulators/src/meson.build
@@ -15,6 +15,7 @@
     'id_map.cpp',
     'journal.cpp',
     'pmbus_utils.cpp',
+    'presence_detection.cpp',
     'presence_service.cpp',
     'rail.cpp',
     'sensor_monitoring.cpp',