arm64: enable superpage mappings by pmap_mapdev{,_attr}()

In order for pmap_kenter{,_device}() to create superpage mappings,
either 64 KB or 2 MB, pmap_mapdev{,_attr}() must request appropriately
aligned virtual addresses.

Reviewed by:	markj
Tested by:	gallatin
Differential Revision:	https://reviews.freebsd.org/D42737

sys/kern/subr_devmap.c

@@ -273,6 +273,13 @@ pmap_mapdev(vm_paddr_t pa, vm_size_t size)
 		KASSERT(va >= VM_MAX_KERNEL_ADDRESS - PMAP_MAPDEV_EARLY_SIZE,
 		    ("Too many early devmap mappings"));
 	} else
+#endif
+#ifdef __aarch64__
+	if (size >= L2_SIZE && (pa & L2_OFFSET) == 0)
+		va = kva_alloc_aligned(size, L2_SIZE);
+	else if (size >= L3C_SIZE && (pa & L3C_OFFSET) == 0)
+		va = kva_alloc_aligned(size, L3C_SIZE);
+	else
 #endif
 		va = kva_alloc(size);
 	if (!va)
@@ -304,6 +311,13 @@ pmap_mapdev_attr(vm_paddr_t pa, vm_size_t size, vm_memattr_t ma)
 		KASSERT(va >= (VM_MAX_KERNEL_ADDRESS - (PMAP_MAPDEV_EARLY_SIZE)),
 		    ("Too many early devmap mappings 2"));
 	} else
+#ifdef __aarch64__
+	if (size >= L2_SIZE && (pa & L2_OFFSET) == 0)
+		va = kva_alloc_aligned(size, L2_SIZE);
+	else if (size >= L3C_SIZE && (pa & L3C_OFFSET) == 0)
+		va = kva_alloc_aligned(size, L3C_SIZE);
+	else
+#endif
 		va = kva_alloc(size);
 	if (!va)
 		panic("pmap_mapdev: Couldn't alloc kernel virtual memory");