usb: gadget: aspeed: read vhub properties from device tree

The patch introduces 2 DT properties ("aspeed,vhub-downstream-ports" and
"aspeed,vhub-generic-endpoints") which replaces hardcoded port/endpoint
number. It is to make it more convenient to add support for newer vhub
revisions with different number of ports and endpoints.

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>