ARM: OMAP: Move plat/omap-serial.h to include/linux/platform_data/serial-omap.h

We need to move this file to allow ARM multiplatform configurations
to build for omap2+. This can now be done as this file now only
contains platform_data.

cc: Russell King <linux@arm.linux.org.uk>
cc: Alan Cox <alan@linux.intel.com>
cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
cc: Govindraj.R <govindraj.raja@ti.com>
cc: Kevin Hilman <khilman@ti.com>
cc: linux-serial@vger.kernel.org
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>