arm64: check return value of of_flat_dt_get_machine_name

It's useless to print machine name and setup arch-specific system
identifiers if of_flat_dt_get_machine_name() return NULL, especially
when ACPI-based boot.
Reviewed-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/setup.c

@@ -194,6 +194,9 @@ static void __init setup_machine_fdt(phys_addr_t dt_phys)
 	}
 
 	name = of_flat_dt_get_machine_name();
+	if (!name)
+		return;
+
 	pr_info("Machine model: %s\n", name);
 	dump_stack_set_arch_desc("%s (DT)", name);
 }