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!

lib/efm32/common/gpio_common.c

@@ -1,3 +1,6 @@
+/** @addtogroup gpio_file GPIO peripheral API
+ * @ingroup peripheral_apis
+ */
 /*
  * This file is part of the libopencm3 project.
  *
@@ -19,6 +22,8 @@
 
 #include <libopencm3/efm32/gpio.h>
 
+/**@{*/
+
 /**
  * Enable GPIO registers lock.
  * @see gpio_disable_lock()
@@ -173,3 +178,5 @@ void gpio_port_config_lock(uint32_t gpio_port, uint16_t gpios)
 {
 	GPIO_P_PINLOCKN(gpio_port) = ~gpios;
 }
+
+/**@}*/