x86, mce: Move debugfs mce dir creating to mce.c

Because more debugfs files under mce dir will be create in mce.c.

ChangeLog:

v5:

- Rebased on x86-tip.git/mce

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>