ARM: dts: aspeed: Enable vhub on port A of AST2500 EVB

This is an eval board, it makes sense to enable many
functions by default. This changes the device-tree to
set port A to be a USB device and leave port B as a
host, along with a little comment explaining how to
change it.

(the vhub device can only exist on port A on this SoC)

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>