ARM: OMAP: move OMAP USB platform data to <linux/platform_data/omap-usb.h>

In order to make single zImage work for ARM architecture,
we need to make sure we don't depend on private headers.

Move USB platform_data to <linux/platform_data/omap-usb.h>
and add a minimal drivers/mfd/usb-omap.h.

Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Partha Basak <parthab@india.ti.com>
Cc: Keshava Munegowda <keshava_mgowda@ti.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Felipe Balbi <balbi@ti.com>
[tony@atomide.com: updated for local mfd/usb-omap.h]
Signed-off-by: Tony Lindgren <tony@atomide.com>