Documentation: bindings: add doc for the USB2 ChipIdea USB driver

Document the USB2 ChipIdea driver (ci13xxx) bindings.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>