serial: omap: Only allow if 8250_omap is not selected

serial: omap: Only allow if 8250_omap is not selected

For years we've been carrying legacy omap-serial in addition to
8250_omap driver and 8250_omap should be used instead.

Let's finally start planning on removing omap-serial by first not
building it if 8250_omap is selected to save some memory.

The defconfigs have switched over to using 8250_omap, and we have
a fixup in place for the the serial console since commit 00648d0282dc
("tty: serial: 8250: omap: add ttySx console if the user didn't").
So people updating their systems without omap-serial will see
boot time warnings on what is going on.

Cc: Andy Shevchenko <andriy.shevchenko@intel.com>
Cc: Dario Binacchi <dariobin@libero.it>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20210727103149.51175-2-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>