x86/intel_rdt: Documentation for Cache Pseudo-Locking

Add description of Cache Pseudo-Locking feature, its interface, as well as
an example of its usage.
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: fenghua.yu@intel.com
Cc: tony.luck@intel.com
Cc: vikas.shivappa@linux.intel.com
Cc: gavin.hindman@intel.com
Cc: jithu.joseph@intel.com
Cc: dave.hansen@intel.com
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/6e118c15d2c254a27b8891783505cd1bb94a2b10.1529706536.git.reinette.chatre@intel.com