Changed to use accessors instead of casting to volatile pointers.

In places where we were defining memory mapped peripheral buffers we
were using directly a cast to "volatile int_type *". For consistency we
should use dereferenced accessor like: &MMIO32(address)

include/libopencm3/cm3/fpb.h

@@ -33,7 +33,7 @@
 #define FPB_REMAP			MMIO32(FPB_BASE + 4)
 
 /* Flash Patch Comparator (FPB_COMPx) */
-#define FPB_COMP			(volatile uint32_t *)(FPB_BASE + 8)
+#define FPB_COMP			(&MMIO32(FPB_BASE + 8))
 
 /* TODO: PID, CID */