x86/tboot: Provide debugfs interfaces to access TXT log

These logs come from tboot (Trusted Boot, an open source,
pre-kernel/VMM module that uses Intel TXT to perform a
measured and verified launch of an OS kernel/VMM.).

Signed-off-by: Qiaowei Ren <qiaowei.ren@intel.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Cc: Gang Wei <gang.wei@intel.com>
Link: http://lkml.kernel.org/r/1372053333-21788-1-git-send-email-qiaowei.ren@intel.com


[ Beautified the code a bit. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>