x86/resctrl: Document the new "mba_MBps_event" file

Add a section to document a new read/write file that shows/sets the memory
bandwidth event used to control bandwidth used by each CTRL_MON group.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Link: https://lore.kernel.org/r/20241206163148.83828-9-tony.luck@intel.com