x86: broadwell: Correct I/O APIC ID

Currently ID 2 is assgined to broadwell I/O APIC, however per
chromebook_samus.dts 2 is the core#2 LAPIC ID. Now we change
I/O APIC ID to 4 to avoid conflict.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>