vf6xx: initial GPIO support

This adds GPIO module support. GPIO can be controlled using the GPIO
number as stated in the reference manual, similar to Linux. Also
32-bit access to whole ports is possible. Reading a GPIO is possible
without muxing the pad as GPIO, however writing a GPIO needs the pad
to be muxed as GPIO.

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
+OBJS		= ccm.o uart.o gpio.o
 
 VPATH += ../cm3