mb/google/poppy/variants/nami: Enable mbox command for ISL VR c-state issue

There is a potential IMVP8 issue for KBL that affects Intersil VRs
Nami is using one of the affected parts. The fix is to use an updated
microcode and also send a mailbox box command from FSP.

BUG=b:112081534
BRANCH=None
TEST=Build and boot Nami
     Verify that suspend/resume and consecutive reboots are working

Change-Id: I6ec18a4c3fae6a66cf8a95685d91a8ba51e2697c
Signed-off-by: Shelley Chen <shchen@google.com>
Reviewed-on: https://review.coreboot.org/27780
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/mainboard/google/poppy/variants/nami/devicetree.cb

@@ -58,6 +58,10 @@ chip soc/intel/skylake
 	register "PmTimerDisabled" = "1"
 	register "VmxEnable" = "1"
 
+	# Intersil VR c-state issue workaround
+	# send VR mailbox command for IA/GT/SA rails
+	register "IslVrCmd" = "2"
+
 	register "pirqa_routing" = "PCH_IRQ11"
 	register "pirqb_routing" = "PCH_IRQ10"
 	register "pirqc_routing" = "PCH_IRQ11"