ARM: EXYNOS: fix build error when MFC is not selected

This fixes following:
arch/arm/mach-exynos/built-in.o: In function `exynos5_reserve':
arch/arm/mach-exynos/mach-exynos5-dt.c:177: undefined reference to `s5p_fdt_find_mfc_mem'
arch/arm/mach-exynos/mach-exynos5-dt.c:177: undefined reference to `s5p_fdt_find_mfc_mem'
arch/arm/mach-exynos/mach-exynos5-dt.c:178: undefined reference to `s5p_mfc_reserve_mem'

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>