ARM: Kirkwood: Convert all DT boards to EHCI via DT.

Now that the EHCI driver has DT support, drop old style configuration
of it and add DT in its place. Since all the boards enable the EHCI,
enable it by default in kirkwood.dtsi. Any new boards which don't have
USB can specifically disable it.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>