net: libwx: add mailbox api for wangxun vf drivers

Implements the mailbox interfaces for Wangxun vf drivers which
will be used in txgbevf and ngbevf.

Signed-off-by: Mengyuan Lou <mengyuanlou@net-swift.com>
Link: https://patch.msgid.link/20250704094923.652-2-mengyuanlou@net-swift.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>