soc/intel/common/dptf: Add method for temp conversion

Add a method to convert from 1/10 Kelvin to Celsius.  This
is useful for EC devices where the sensor temperature are
stored in Celsius instead of Kelvin.

Change-Id: I6b1154f5ba13416131a029966d6d5c1598904881
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/29758
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/common/acpi/dptf/dptf.asl

@@ -95,6 +95,21 @@ Device (DPTF)
 		Return (Local0)
 	}
 
+	/* Convert from 1/10 Kelvin to Degrees C for ACPI */
+	Method (KTOC, 1) {
+		If (LLessEqual (Arg0, 2732)) {
+			Return (0)
+		}
+
+		/* Convert to Celsius */
+		Subtract (Arg0, 2732, Local0)
+
+		/* Convert from 10th of degrees */
+		Divide (Local0, 10,, Local0)
+
+		Return (Local0)
+	}
+
 	/* Include Thermal Participants */
 	#include "thermal.asl"