x86/intel_rdt: Add info files to Documentation

The content of the directories and files in the info directory of the
resctrl filesystem are not documented.

Add the missing bits and pieces.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Shaohua Li <shli@fb.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Tony Luck <tony.luck@intel.com>