x86/intel_rdt: Introduce resource group's mode resctrl file

A new resctrl file "mode" associated with each resource group is
introduced. This file will display the resource group's current mode and an
administrator can also use it to modify the resource group's mode.

Only shareable mode is currently supported.

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/20ab78fda26a8c8d98e18ec555f6a1f728948972.1529706536.git.reinette.chatre@intel.com