Name each ehci driver uniquely.
This remove the warning printed at each arm boot :
module_register: cannot register simplebus/ehci from kernel; already loaded from kernel
A similar fix was done in r333074 but imx_ehci wasn't renamed and generic_ehci wasn't
present at that time.
Create an attachment file for the existing ACPI attachment, and create a
new FDT attachment for the generic-ehci driver.
Submitted by: andrew (Original version)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D19389