omap: update plat/usb.h to allow ehci driver to build

Add missing declarations to allow the recently introduced
ehci-omap driver to build on OMAP3

Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Cc: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>