The generic EHCI binding needs to document all the specific compatible
strings so we can track undocumented compatible strings. Add all the
compatible strings from in tree users.

Turns out we also have the generic 'usb-ehci' compatible which is pretty
much the same binding and the correct one for the example, so let's add it.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210202175439.3904060-1-robh@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>