usb: host: ohci-platform: Add basic runtime PM support

This is needed in preparation of adding support for omap3 and
later OHCI. The runtime PM will only do something on platforms
that implement it.

Cc: devicetree@vger.kernel.org
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Cc: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>