x86/Documentation: Start documenting x86 topology

This should contain important aspects of how we represent the system
topology on x86. If people have questions about it and this file doesn't
answer it, then it must be updated.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/20160328095609.GD26651@pd.tnic
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>