Create stubs for regulator Device, Rail, and Rule

Create stub versions of the Device, Rail, and Rule classes for the
phosphor-regulators application.  Having a stub version of these classes
is a pre-requisite for implementing the action framework.

Also updated the top level meson.build file to build the
phosphor-regulators directory.  Created new meson.build files for
several phosphor-regulators sub-directories.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I50b018db744191acd1e473652019906a8506745f
diff --git a/meson.build b/meson.build
index a31edac..8567052 100644
--- a/meson.build
+++ b/meson.build
@@ -76,6 +76,7 @@
     ],
 )
 
+subdir('phosphor-regulators')
 subdir('power-sequencer')
 subdir('power-supply')
 subdir('tools/power-utils')