x86/intel_rdt: Introduce a common compile option for RDT

We currently have a CONFIG_RDT_A which is for RDT(Resource directory
technology) allocation based resctrl filesystem interface. As a
preparation to add support for RDT monitoring as well into the same
resctrl filesystem, change the config option to be CONFIG_RDT which
would include both RDT allocation and monitoring code.

No functional change.

Signed-off-by: Vikas Shivappa <vikas.shivappa@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: ravi.v.shankar@intel.com
Cc: tony.luck@intel.com
Cc: fenghua.yu@intel.com
Cc: peterz@infradead.org
Cc: eranian@google.com
Cc: vikas.shivappa@intel.com
Cc: ak@linux.intel.com
Cc: davidcc@google.com
Cc: reinette.chatre@intel.com
Link: http://lkml.kernel.org/r/1501017287-28083-4-git-send-email-vikas.shivappa@linux.intel.com