omap2plus: omap4: Set NR_CPU to 2 instead of default 4

The omap2plus_defconfig picks default NR_CPU value as 4 which isn't
correct for OMAP4430. Available CPUs are ony 2, so fix the same.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>