ARM: omap2plus_defconfig: Enable 8250_OMAP

We've had 8250_OMAP working for few years now, so let's start enabling
it by default. We provide users an option to warn by default with
SERIAL_8250_OMAP_TTYO_FIXUP by default. That option updates the serial
console to point to 8250 driver and warns about it.

Users with ttyO[0123] in their /etc/inittab should update inittab
to use ttyS[0123] instead.

Signed-off-by: Tony Lindgren <tony@atomide.com>