LoongArch: Clean up plat_swiotlb_setup() related code

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>

arch/loongarch/include/asm/bootinfo.h

@@ -13,7 +13,6 @@ const char *get_system_type(void);
 extern void init_environ(void);
 extern void memblock_init(void);
 extern void platform_init(void);
-extern void plat_swiotlb_setup(void);
 extern int __init init_numa_memory(void);
 
 struct loongson_board_info {

arch/loongarch/kernel/setup.c

@@ -389,8 +389,8 @@ static void __init arch_mem_init(char **cmdline_p)
 	/*
 	 * In order to reduce the possibility of kernel panic when failed to
 	 * get IO TLB memory under CONFIG_SWIOTLB, it is better to allocate
-	 * low memory as small as possible before plat_swiotlb_setup(), so
-	 * make sparse_init() using top-down allocation.
+	 * low memory as small as possible before swiotlb_init(), so make
+	 * sparse_init() using top-down allocation.
 	 */
 	memblock_set_bottom_up(false);
 	sparse_init();