Add memmmap on arm64 so we can mmap /dev/mem and /dev/kmem.

Obtained from:	ABT Systems Ltd
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

sys/arm64/arm64/mem.c

@@ -114,3 +114,19 @@ memrw(struct cdev *dev, struct uio *uio, int flags)
 	return (error);
 }
 
+/*
+ * allow user processes to MMAP some memory sections
+ * instead of going through read/write
+ */
+/* ARGSUSED */
+int
+memmmap(struct cdev *dev, vm_ooffset_t offset, vm_paddr_t *paddr,
+    int prot __unused, vm_memattr_t *memattr __unused)
+{
+	if (dev2unit(dev) == CDEV_MINOR_MEM)
+		*paddr = offset;
+	else if (dev2unit(dev) == CDEV_MINOR_KMEM)
+		*paddr = vtophys(offset);
+	/* else panic! */
+	return (0);
+}

sys/arm64/include/memdev.h

@@ -35,6 +35,6 @@
 d_open_t	memopen;
 d_read_t	memrw;
 #define		memioctl	(d_ioctl_t *)NULL
-#define		memmmap		(d_mmap_t *)NULL
+d_mmap_t	memmmap;
 
 #endif /* _MACHINE_MEMDEV_H_ */