usb: host: Allow to build ehci orion with mvebu SoCs

The mvebu ARM64 SoCs no longer select PLAT_ORION. However Armada 37xx use
the Orion EHCI controller. This patch allows the Orion EHCI driver to be
built when ARCH_MVEBU is selected.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>