x86/MCE: Cleanup and complete struct mce fields definitions

The struct is part of the uapi, document that fact and all fields properly
and fix formatting.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20180306142143.19990-3-bp@alien8.de