dm: usb: udc: Use SEQ_ALIAS to index the USB gadget ports

dfu, fastbot and other usb gadget commands take the USB port index as a
parameter. Currently this index is assigned in the order of the driver
bindings.
Changing this behavior using the SEQ_ALIAS feature. This option assign to
the device a SEQ number based on its alias (if it exists)

To use it we must set the DM_UC_FLAG_SEQ_ALIAS flag and follow the existing
naming convention: use "usb" for the name of the gadget UCLASS_DRIVER
(same as for the UCLASS_USB).

If no alias is provided, then the index falls back to the order in which
the bindings took place.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reported-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Tested-by: Sam Protsenko <semen.protsenko@linaro.org>