fs/resctrl: Introduce the interface to display monitoring modes

Introduce the resctrl file "mbm_assign_mode" to list the supported counter
assignment modes.

The "mbm_event" counter assignment mode allows users to assign a hardware
counter to an RMID, event pair and monitor bandwidth usage as long as it is
assigned. The hardware continues to track the assigned counter until it is
explicitly unassigned by the user. Each event within a resctrl group can be
assigned independently in this mode.

On AMD systems "mbm_event" mode is backed by the ABMC (Assignable Bandwidth
Monitoring Counters) hardware feature and is enabled by default.

The "default" mode is the existing mode that works without the explicit
counter assignment, instead relying on dynamic counter assignment by hardware
that may result in hardware not dedicating a counter resulting in monitoring
data reads returning "Unavailable".

Provide an interface to display the monitor modes on the system.

  $ cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
  [mbm_event]
  default

Add IS_ENABLED(CONFIG_RESCTRL_ASSIGN_FIXED) check to support Arm64.

On x86, CONFIG_RESCTRL_ASSIGN_FIXED is not defined. On Arm64, it will be
defined when the "mbm_event" mode is supported.

Add IS_ENABLED(CONFIG_RESCTRL_ASSIGN_FIXED) check early to ensure the user
interface remains compatible with upcoming Arm64 support. IS_ENABLED() safely
evaluates to 0 when the configuration is not defined.

As a result, for MPAM, the display would be either:

  [default]

or

  [mbm_event]

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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