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: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>