usb: add ehci adapter

usb: add ehci adapter

This patch finally merges the EHCI host adapter aka USB 2.0 support.

Based on the ehci bits collected @ git://git.kiszka.org/qemu.git ehci

EHCI has a long out-of-tree history.  Project was started by Mark
Burkley, with contributions by Niels de Vos.  David S. Ahern continued
working on it.  Kevin Wolf, Jan Kiszka and Vincent Palatin contributed
bugfixes.

/me (Gerd Hoffmann) picked it up where it left off, prepared the code
for merge, fixed a few bugs and added basic user docs.

Cc: David S. Ahern <daahern@cisco.com>
Cc: Jan Kiszka <jan.kiszka@web.de>
Cc: Kevin Wolf <mail@kevin-wolf.de>
Cc: Vincent Palatin <vincent.palatin_qemu@m4x.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>