Add a generic EHCI USB driver based on the Allwinner A10 driver. It is ACPI
only for now, but wouldn't be too difficult to add support for FDT.

Reviewed by: hselasky
Obtained from: ABT Systems Ltd
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7352