acpi: get boot_cpu_id as early for k8_scan_nodes

[mingo@elte.hu: split from "x86_64: get boot_cpu_id as early for k8_scan_nodes]

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>