regulators: Add PMBus utilities

Create namespace pmbus_utils.  This namespace contains utilities for
sending PMBus commands over an I2C interface.

These utilities are needed to implement the pmbus_write_vout_command and
pmbus_read_sensor actions from the JSON config file.

See pmbus_write_vout_command.md and pmbus_read_sensor.md for more
information on these actions.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I2fabca3ac8b9cd8b1f2b462c80feef6b6d7d60e8
diff --git a/phosphor-regulators/test/meson.build b/phosphor-regulators/test/meson.build
index 707bc83..78661c5 100644
--- a/phosphor-regulators/test/meson.build
+++ b/phosphor-regulators/test/meson.build
@@ -6,6 +6,7 @@
 phosphor_regulators_tests_source_files = [
     'device_tests.cpp',
     'id_map_tests.cpp',
+    'pmbus_utils_tests.cpp',
     'rail_tests.cpp',
     'rule_tests.cpp',