ARM: OMAP2+: Add missing CONFIG_ARCH_MULTI_V6=y to omap2plus_defconfig

omap2plus_defconfig is missing CONFIG_ARCH_MULTI_V6.  This results in
undefined instruction traps in u-boot (and boot failures) on OMAP2xxx
SoCs, which are ARM11-based.  Fix by setting CONFIG_ARCH_MULTI_V6.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>