fs/resctrl: Introduce the interface to switch between monitor modes

Resctrl subsystem can support two monitoring modes, "mbm_event" or "default".
In mbm_event mode, monitoring event can only accumulate data while it is
backed by a hardware counter. In "default" mode, resctrl assumes there is
a hardware counter for each event within every CTRL_MON and MON group.

Introduce mbm_assign_mode resctrl file to switch between mbm_event and default
modes.

Example:
To list the MBM monitor modes supported:
  $ cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
  [mbm_event]
  default

To enable the "mbm_event" counter assignment mode:
  $ echo "mbm_event" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode

To enable the "default" monitoring mode:
  $ echo "default" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode

Reset MBM event counters automatically as part of changing the mode.  Clear
both architectural and non-architectural event states to prevent overflow
conditions during the next event read. Clear assignable counter configuration
on all the domains. Also, enable auto assignment when switching to "mbm_event"
mode.

Signed-off-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/cover.1757108044.git.babu.moger@amd.com

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