psu-ng: Add accessor to get GPIO line name

Update the utilities to give the line name for a GPIO.

Add accessor to power supply class to get the name of the presence GPIO
line.

Signed-off-by: B. J. Wyman <bjwyman@gmail.com>
Change-Id: Ic10fd81c6de024dbcb27088c5935ac9947af9465
diff --git a/phosphor-power-supply/util.hpp b/phosphor-power-supply/util.hpp
index 2203b92..da04020 100644
--- a/phosphor-power-supply/util.hpp
+++ b/phosphor-power-supply/util.hpp
@@ -121,6 +121,11 @@
      */
     int read() override;
 
+    /**
+     * @brief Returns the name of the GPIO, if not empty.
+     */
+    std::string getName() const override;
+
   private:
     gpiod::line line;
 };