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.cpp b/phosphor-power-supply/util.cpp
index f80bac7..5942ce7 100644
--- a/phosphor-power-supply/util.cpp
+++ b/phosphor-power-supply/util.cpp
@@ -31,6 +31,11 @@
     return std::make_unique<GPIOReader>(namedGpio);
 }
 
+std::string GPIOReader::getName() const
+{
+    return line.name();
+}
+
 int GPIOReader::read()
 {
     using namespace phosphor::logging;