mb/intel/mtlrvp: Disable package C-state auto demotion

Package C-state auto demotion feature allows hardware to determine lower
C-state as per platform policy. Since platform sets performance policy
to balanced from hardware, auto demotion can be disabled without
performance impact.

Also, disabling this feature results soc to enter below PC8 state and
additional power savings ~30mW in Local-Video-Playback scenario.

Change-Id: I6ff408280178a24686180f72f79522d2741607a1
Signed-off-by: Sukumar Ghorai <sukumar.ghorai@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78278
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/intel/mtlrvp/variants/baseboard/mtlrvp_p/devicetree.cb

@@ -55,6 +55,9 @@ chip soc/intel/meteorlake
 	# Disable C1 C-state auto-demotion
 	register "disable_c1_state_auto_demotion" = "1"
 
+	# Disable PKGC-state demotion
+	register "disable_package_c_state_demotion" = "1"
+
 	# Enable energy reporting
 	register "pch_pm_energy_report_enable" = "1"