Partly revert r295168 and define PTE_DEVICE in pmap-v6.h header again.

It turned out that devmap.c is not only file in which PTE_DEVICE
is used and simultaneously, built for both armv4 and armv6 platforms.

When I tried to build all arm kernels before r295168 commit, it was
hid by some other local changes in my tree. I hope that this is just
temporary workaround before VM_MEMATTR_DEVICE could be used instead of
PTE_DEVICE outside of pmap code for __ARM_ARCH < 6.

sys/arm/arm/devmap.c

@@ -55,9 +55,6 @@ static boolean_t devmap_bootstrap_done = false;
 #define	PTE_DEVICE	VM_MEMATTR_DEVICE
 #elif defined(__arm__)
 #define	MAX_VADDR	ARM_VECTORS_HIGH
-#if __ARM_ARCH >= 6
-#define	PTE_DEVICE	VM_MEMATTR_DEVICE
-#endif
 #endif
 
 /*

sys/arm/include/pmap-v6.h

@@ -250,6 +250,8 @@ void pmap_preboot_map_attr(vm_paddr_t, vm_offset_t, vm_size_t, vm_prot_t,
  */
 void vector_page_setprot(int);
 
+#define PTE_DEVICE	VM_MEMATTR_DEVICE
+
 #endif	/* _KERNEL */
 // -----------------------------------------------------------------------------