usb: host: ohci-st: Add ohci-st devicetree bindings documentation

This patch documents the device tree bindings required for
the ohci on-chip controller found in ST consumer electronics SoC's.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>