USB: EHCI: DT support for generic bus glue

This lets us use the ehci-platform driver on platforms without special
requirements for their ehci controllers. In particular, this is true
for the vt8500/wm8x50 platforms, which currently have a separate
driver that causes problems with multiplatform configurations.

Tested-by: Tony Prisk <linux@prisktech.co.nz>
Tested-by: Peter Vasil <petervasil@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>