ARM: exynos_defconfig: increase CONFIG_NR_CPUS value to 8

Current CONFIG_NR_CPUS value (2) is too small for:
- EXYNOS4412 (4 cores)
- EXYNOS5440 (4 cores)
- EXYNOS5410 (8 cores)
- EXYNOS5420 (8 cores)

Set CONFIG_NR_CPUS to 8 so it is correct for all currently
supported SoCs.

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>