firmware: memmap: Constify memmap_ktype

This 'memmap_ktype' is not modified. It is only used in
firmware_map_add_entry().

Constifying this structure and moving it to a read-only section,
and this can increase over all security.

```
[Before]
text data bss dec hex filename
4345 596 12 4953 1359 drivers/firmware/memmap.o

[After]
text data bss dec hex filename
4393 548 12 4953 1359 drivers/firmware/memmap.o
```

Signed-off-by: Kunwu Chan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/firmware/memmap.c

···
 	kfree(entry);
 }
 
-static struct kobj_type __refdata memmap_ktype = {
+static const struct kobj_type memmap_ktype = {
 	.release	= release_firmware_map_entry,
 	.sysfs_ops	= &memmap_attr_ops,
 	.default_groups	= def_groups,