psu-ng: Add code to set ON_OFF_CONFIG

Add code that sends the appropriate ON_OFF_CONFIG mask for the power
supply power on operation.

Tested:
    Ran gtest using x86sdk
    Used simulator to change ON_OFF_CONFIG value, restarted service,
    verified ON_OFF_CONFIG changed.
    Used i2cget on hardware to change ON_OFF_CONFIG, restarted service,
    verified ON_OFF_CONFIG changed.

Signed-off-by: Brandon Wyman <bjwyman@gmail.com>
Change-Id: Ifd9d63fcd2e86a62b7358e08958b5e2dbd21db9f
diff --git a/phosphor-power-supply/power_supply.hpp b/phosphor-power-supply/power_supply.hpp
index 707a745..487fa53 100644
--- a/phosphor-power-supply/power_supply.hpp
+++ b/phosphor-power-supply/power_supply.hpp
@@ -65,6 +65,16 @@
     void analyze();
 
     /**
+     * Write PMBus ON_OFF_CONFIG
+     *
+     * This function will be called to cause the PMBus device driver to send the
+     * ON_OFF_CONFIG command. Takes one byte of data.
+     *
+     * @param[in] data - The ON_OFF_CONFIG data byte mask.
+     */
+    void onOffConfig(uint8_t data);
+
+    /**
      * Write PMBus CLEAR_FAULTS
      *
      * This function will be called in various situations in order to clear