x86: Save mtrr support flag in global data

CPUID (EAX 01H) returns MTRR support flag in EDX bit 12. Probe this
flag in x86_cpu_init_f() and save it in global data.

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