x86/resctrl: Prepare for per-CTRL_MON group mba_MBps control

Resctrl uses local memory bandwidth event as input to the feedback loop when
the mba_MBps mount option is used. This means that this mount option cannot be
used on systems that only support monitoring of total bandwidth.

Prepare to allow users to choose the input event independently for each
CTRL_MON group by adding a global variable "mba_mbps_default_event" used to
set the default event for each CTRL_MON group, and a new field
"mba_mbps_event" in struct rdtgroup to track which event is used for each
CTRL_MON group.

Notes:

1) Both of these are only used when the user mounts the filesystem with the
   "mba_MBps" option.
2) Only check for support of local bandwidth event when initializing
   mba_mbps_default_event. Support for total bandwidth event can be added
   after other routines in resctrl have been updated to handle total bandwidth
   event.

  [ bp: Move mba_mbps_default_event extern into the arch header. ]

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/20241206163148.83828-3-tony.luck@intel.com

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