arm: configs: Add USB gadget to Aspeed G4 defconfig

Now that the vhub driver is upstream and the device-trees
updated, let's enable this by default.

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>