fs/resctrl: Introduce mbm_L3_assignments to list assignments in a group

fs/resctrl: Introduce mbm_L3_assignments to list assignments in a group

Introduce the mbm_L3_assignments resctrl file associated with CTRL_MON and MON
resource groups to display the counter assignment states of the resource group
when "mbm_event" counter assignment mode is enabled.

Display the list in the following format:
<Event>:<Domain id>=<Assignment state>;<Domain id>=<Assignment state>

Event: A valid MBM event listed in
      /sys/fs/resctrl/info/L3_MON/event_configs directory.

Domain ID: A valid domain ID.

The assignment state can be one of the following:

_ : No counter assigned.

e : Counter assigned exclusively.

Example:
To list the assignment states for the default group
  $ cd /sys/fs/resctrl
  $ cat /sys/fs/resctrl/mbm_L3_assignments
  mbm_total_bytes:0=e;1=e
  mbm_local_bytes:0=e;1=e

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