kernel: fix usb chipidea controller support

If ehci platform driver is loaded before the chipidea controller driver,
both are competing for the same IO resources and the generic driver gets
used for the hardware. This results in USB device mode being
unavailable.
Split generic EHCI support code out of kmod-usb2, so that the chipidea
driver can be included without also pulling in the generic one. Also
rework the load order, so that the chipidea driver gets loaded first, in
case both are installed

Signed-off-by: Felix Fietkau <nbd@nbd.name>