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/test/mock.hpp b/phosphor-power-supply/test/mock.hpp
index 2731692..d494e14 100644
--- a/phosphor-power-supply/test/mock.hpp
+++ b/phosphor-power-supply/test/mock.hpp
@@ -54,6 +54,7 @@
 {
   public:
     MOCK_METHOD(int, read, (), (override));
+    MOCK_METHOD(std::string, getName, (), (const, override));
 };
 
 const UtilBase& getUtils();