After commit c78c43fe ("LoongArch: Use acpi_arch_dma_setup() and
remove ARCH_HAS_PHYS_TO_DMA"), plat_swiotlb_setup() has been deleted,
so clean up the related code.
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>