mb/google/poppy: Set UPD CmdTriStateDis for Nocturne

This patch sets the MRC UPD CmdTriStateDis for the
nocturne boards.Nocturne is LPDDR3 design without RTT
for CMD/CTRL.

BUG=b:111812662
TEST=Run memtester app and also webgl fishtank on
     the LPDDR3 kabylake boards and also check the
     margin data is proper in FSP.

Change-Id: I0f593761dcbd121e7e758421af178931b9d78295
Signed-off-by: Shaunak Saha <shaunak.saha@intel.com>
Reviewed-on: https://review.coreboot.org/28379
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Nick Vaccaro <nvaccaro@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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

@@ -29,6 +29,9 @@ chip soc/intel/skylake
 	# Enable S0ix
 	register "s0ix_enable" = "1"
 
+	# Disable Command TriState
+	register "CmdTriStateDis" = "1"
+
 	# FSP Configuration
 	register "ProbelessTrace" = "0"
 	register "EnableLan" = "0"