x86: Move MP initialization codes into a common place

x86: Move MP initialization codes into a common place

Most of the MP initialization codes in arch/x86/cpu/baytrail/cpu.c is
common to all x86 processors, except detect_num_cpus() which varies
from cpu to cpu. Move these to arch/x86/cpu/cpu.c and implement the
new 'get_count' method for baytrail and cpu_x86 drivers. Now we call
cpu_get_count() in mp_init() to get the number of CPUs.

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