x86/intel_rdt: Document new mode, size, and bit_usage

By default resource groups allow sharing of their cache allocations.  There
is nothing that prevents a resource group from configuring a cache
allocation that overlaps with that of an existing resource group.

To enable resource groups to specify that their cache allocations cannot be
shared a resource group "mode" is introduced to support two possible modes:
"shareable" and "exclusive". A "shareable" resource group allows sharing of
its cache allocations, an "exclusive" resource group does not. A new
resctrl file "mode" associated with each resource group is used to
communicate its (the associated resource group's) mode setting and allow
the mode to be changed.  The new "mode" file as well as two other resctrl
files, "bit_usage" and "size", are introduced in this series.

Add documentation for the three new resctrl files as well as one example
demonstrating their use.

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/f03a3059ec40ae719be6f3fba9f446bb055e0064.1529706536.git.reinette.chatre@intel.com