dt-bindings: usb: Convert the generic EHCI binding to YAML

The generic EHCI binding is used by many controllers that are using the
EHCI spec.

Convert that binding to a YAML description to enable the validation on all
the nodes using that binding.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>