x86/mm/pat: Add comments to cachemode translation tables

Add comments to the cachemode translation tables to clarify that
the default values are set as minimal supported mode, which are
necessary to handle WC and WT fallback to UC- when they are not
enabled.

Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1437588371-28223-1-git-send-email-toshi.kani@hp.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>