doc: efm32: i2c: tag for doxygen

Includes a stub .c file until there are some APIs

lib/efm32/common/i2c_common.c

@@ -0,0 +1,15 @@
+/** @addtogroup i2c_file I2C peripheral API
+ * @ingroup peripheral_apis
+ * @brief I²C helper functions.
+ *
+ * <b>NO</b> helper functions exist. Only header definitions are available.
+ * Delete these lines if/when you add actual helper APIs.
+ * @copyright See @ref lgpl_license
+ */
+
+#include <libopencm3/efm32/i2c.h>
+
+/**@{*/
+
+/**@}*/
+

lib/efm32/ezr32wg/Makefile

@@ -45,6 +45,7 @@ OBJS += dac_common.o
 OBJS += dma_common.o
 OBJS += emu_common.o
 OBJS += gpio_common.o
+OBJS += i2c_common.o
 OBJS += prs_common.o
 OBJS += timer_common.o
 

lib/efm32/lg/Makefile

@@ -45,6 +45,7 @@ OBJS += dac_common.o
 OBJS += dma_common.o
 OBJS += emu_common.o
 OBJS += gpio_common.o
+OBJS += i2c_common.o
 OBJS += prs_common.o
 OBJS += timer_common.o
 

lib/efm32/wg/Makefile

@@ -45,6 +45,7 @@ OBJS += dac_common.o
 OBJS += dma_common.o
 OBJS += emu_common.o
 OBJS += gpio_common.o
+OBJS += i2c_common.o
 OBJS += prs_common.o
 OBJS += timer_common.o