ARM: tegra: Remove useless usb-ehci compatible string

There's no such thing as a generic USB EHCI controller. The EHCI
controllers found on Tegra SoCs are instantiations that need Tegra-
specific glue to work properly, so drop the generic compatible string
and keep only the Tegra-specific ones.

Signed-off-by: Thierry Reding <treding@nvidia.com>