regulators: Create PresenceDetection class

Create C++ class that implements the 'presence_detection' element from
the JSON config file.  See presence_detection.md for more information.

The execute() method of this class will be implemented in a future
commit.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: Id6e1d66afe9179c92455d3f7b8f15bb54745f3eb
diff --git a/phosphor-regulators/test/meson.build b/phosphor-regulators/test/meson.build
index 0220532..38c038d 100644
--- a/phosphor-regulators/test/meson.build
+++ b/phosphor-regulators/test/meson.build
@@ -9,6 +9,7 @@
     'id_map_tests.cpp',
     'pmbus_error_tests.cpp',
     'pmbus_utils_tests.cpp',
+    'presence_detection_tests.cpp',
     'rail_tests.cpp',
     'rule_tests.cpp',
     'write_verification_error_tests.cpp',