ARM: omap2plus_defconfig: Enable ARM erratum 430973 for omap3

Somehow we don't have this set in omap2plus_defconfig. Without this
apps can segfault randomly on omap3. I can reproduce this easily
on am37xx-evm by doing apt-get update over NFSroot.

Tested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>