stm32: dcmi: hook up doxygen again

We still need stub .c files as we're using shared common files, so .d
tracking doesn't work.  It lets us setup basic introductory text anyway,
and there will be .c files eventually, so acceptable.

lib/stm32/f4/Makefile

@@ -42,6 +42,7 @@ OBJS += can.o
 OBJS += crc_common_all.o
 OBJS += crypto_common_f24.o crypto.o
 OBJS += dac_common_all.o
+OBJS += dcmi_common_f47.o
 OBJS += desig_common_all.o desig_common_v1.o
 OBJS += dma_common_f24.o
 OBJS += dma2d_common_f47.o