vf6xx: add I/O mux controller

Add I/O mux controller which is required to mux pins according to
their used function. For all pads, the alternative function 0 is
GPIO. For different pin mux function refer to the reference manual.

lib/vf6xx/Makefile

@@ -34,7 +34,7 @@ CFLAGS		= -Os -g \
 		  -mcpu=cortex-m4 -mthumb $(FP_FLAGS) -Wstrict-prototypes \
 		  -ffunction-sections -fdata-sections -MD -DVF6XX
 ARFLAGS		= rcs
-OBJS		= ccm.o uart.o gpio.o
+OBJS		= ccm.o uart.o gpio.o iomuxc.o
 
 VPATH += ../cm3