doxygen: add groups for all modules

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/max5487/max5487.h

@@ -33,6 +33,11 @@
 
 namespace upm {
 
+/**
+ * @brief max5847 digital potentiometer library
+ * @defgroup max5847 libupm-max5847
+ */
+
 /**
  * @brief C++ API for MAX5487 chip (SPI digital potentionmeter)
  *
@@ -43,6 +48,7 @@ namespace upm {
  * PMOD module](http://datasheets.maximintegrated.com/en/ds/MAX5487PMB1.pdf)
  * from the analog PMOD kit.
  *
+ * @ingroup max5847
  * @snippet max5487.cxx Interesting
  * @image html max5487.jpeg
  */