soc/intel/skylake: Perform PCR read after all PCR write

BIOS must ensure to read same PCR offset after PCR write operation
is done.

BUG=b:35587084
BRANCH=eve
TEST=manual stress testing of D0<->D3 transition on eve failing
unit. No hard hang with this fix.

Change-Id: Id3d567aab517b16ff99a526fc29c2d71bf4042d0
Signed-off-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-on: https://review.coreboot.org/20461
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/soc/intel/skylake/acpi/pcr.asl

@@ -53,6 +53,14 @@ Method (PCRA, 3, Serialized)
 		DATA, 32
 	}
 	And (DATA, Arg2, DATA)
+
+	/*
+	 * After every write one needs to read an innocuous register
+	 * to ensure the writes are completed for certain ports. This is done
+	 * for all ports so that the callers don't need the per-port knowledge
+	 * for each transaction.
+	 */
+	PCRR (Arg0, Arg1)
 }
 
 /*
@@ -69,4 +77,12 @@ Method (PCRO, 3, Serialized)
 		DATA, 32
 	}
 	Or (DATA, Arg2, DATA)
+
+	/*
+	 * After every write one needs to read an innocuous register
+	 * to ensure the writes are completed for certain ports. This is done
+	 * for all ports so that the callers don't need the per-port knowledge
+	 * for each transaction.
+	 */
+	PCRR (Arg0, Arg1)
 }