usbdebug: Support choice of EHCI controller

Nowadays, chipsets or boards do not only have one USB port with the
capabilities of a debug port but several ones. Some of these ports are
easier accessible than others, so making them configurable is also necessary.
This change adds infrastructure to switch between EHCI controllers,
but does not implement it for any chipset.

Change-Id: I079643870104fbc64091a54e1bfd56ad24422c9f
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3438
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>
11 files changed