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 <chentao@kylinos.cn>
Link: https://lore.kernel.org/r/20240924030533.34407-1-chentao@kylinos.cn


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>