usb: gadget: aspeed: support multiple language strings

This patch introduces a link list to store string descriptors with
different languages, and "ast_vhub_rep_string" function is also improved
to support multiple language usb strings.

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>