usb: gadget: aspeed: allow to set usb strings in device tree

If "vhub,string-descriptor" device tree property is defined, the driver
will load string descriptors from device tree; otherwise, the default
string descriptors will be used.

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