drm/xe/hwmon: Add support to manage PL2 though mailbox

drm/xe/hwmon: Add support to manage PL2 though mailbox

Add support to manage power limit PL2 (burst limit) through
pcode mailbox commands.

v2:
 - Update power1_cap definition in hwmon documentation. (Badal)
 - Clamp PL2 power limit to GPU firmware default value.

v3:
 - Activate the power label when either the PL1 or PL2 power
   limit is enabled.

v4:
 - Update description of pl2_on_boot variable to fix kernel-doc
   error.

v5:
 - Remove unnecessary drm_warn.
 - Rectify powerX_label permission to read-only on platforms
   without mailbox power limits support.
 - Expose powerX_cap entries only on platforms with mailbox
   support.

v6:
 - Improve commit message, refer to BIOS as GPU firmware.
 - Refer to card firmware as GPU firmware in code.
 - Remove unnecessary drm_dbg.
 - Print supported and unsupported power limits. (Rodrigo)
 - Enable powerN_cap/max_xxx entries only when power limits
   supported in GPU firmware.

Signed-off-by: Karthik Poosa <karthik.poosa@intel.com>
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Link: https://lore.kernel.org/r/20250529163458.2354509-4-karthik.poosa@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>