usb: Create separate header for ehci-dbgp

The FUSBH200 and FOTG210 controllers implement sufficiently EHCI-
compatible debug ports to leverage ehci-dbgp from their respective
drivers.  Rather than including <linux/usb/ehci_def.h> header, though,
they replicate the necessary declarations in their own headers.  Move
the ehci-dbgp stuff into its own header as a first step towards removing
this redundancy.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>