ARM: multi_v7_defconfig: Enable 8250-omap serial driver and use it by default

We've already moved omap2plus_defconfig over to use 8250-omap instead of
omap-serial driver. Let's update multi_v7_defconfig too.

By default we also enable SERIAL_8250_OMAP_TTYO_FIXUP that updates the
kernel serial console to point to 8250 driver and warns about it during
the boot.

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

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