ARM: dts: UniPhier: add USB EHCI device nodes

No dedicated driver compatible with "socionext,uniphier-ehci" has
been supported in the mainline yet, so it falls into "generic-ehci"
and it works well enough with our USB 2.0 host controllers.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Olof Johansson <olof@lixom.net>