serial: omap: move uart_omap_port definition to C file

nobody needs to access the uart_omap_port structure
other than omap-serial.c file. Let's move that
structure definition to the C source file in order
to prevent anyone from accessing our structure.

Tested-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>