usb: gadget: aspeed: support per-vhub usb descriptors

This patch store vhub's standard usb descriptors in struct "ast_vhub" so
it's more convenient to customize descriptors and potentially support
multiple vhub instances in the future.

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