Documentation: dt-bindings: update ehci-orion binding documentation

This commit updates the Device Tree binding documentation of
ehci-orion to take into account the fact that we can now optionally
pass a clock and a PHY reference.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>