doc: efm32: include gpio_common

gpio_common was built, but wasn't picked up by any documentation.
Use the peripheral_apis style from stm32, but try out the #pragma once
style.  Downside is you don't get warnings if you include an sub layer
.h file.  Upsides are
* no ifdef/endif blocks
* no /**@cond*/ /**@endcond*/ blocks
* that's enough win!

include/libopencm3/efm32/hg/gpio.h

@@ -1,3 +1,11 @@
+/** @defgroup gpio_defines GPIO Defines
+ *
+ * @brief <b>Defined Constants and Types for the EFM32HG GPIO module</b>
+ *
+ * @ingroup EFM32HG_defines
+ *
+ * LGPL License Terms @ref lgpl_license
+ */
 /*
  * This file is part of the libopencm3 project.
  *
@@ -15,9 +23,6 @@
  * along with this library.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef LIBOPENCM3_GPIO_H
-#define LIBOPENCM3_GPIO_H
+#pragma once
 
 #include <libopencm3/efm32/common/gpio_common_hglg.h>
-
-#endif