LoongArch: Adjust arch/loongarch/Kconfig

1, ACPI, EFI and SMP are mandatories for LoongArch, select them
unconditionally to avoid various build errors for 'make randconfig'.
2, Move the MMU_GATHER_MERGE_VMAS selection to the correct place.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>