x86: usb debug port early console, v4

based on work from Eric, and add some timeout so don't dead loop when debug
device is not installed

v2: fix checkpatch warning
v3: move ehci struct def to linux/usrb/ehci_def.h from host/ehci.h
    also add CONFIG_EARLY_PRINTK_DBGP to disable it by default
v4: address comments from Ingo, seperate ehci reg def moving to another patch
    also add auto detect port that connect to debug device for Nvidia
    southbridge

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: "Arjan van de Ven" <arjan@infradead.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Greg KH" <greg@kroah.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>