x86: mce: Split timer init

Split timer init function into the init and the start part, so the
start part can replace the open coded version in CPU_DOWN_FAILED.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Acked-by: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>