broadwell: gpio.asl: Make GWAK method serialized

This method creates named objects and must be serialized
to prevent a warning from IASL.

Tested by compiling purism/librem13 which includes this ASL.

Change-Id: Ic043ea479e681d2180421fcf8e0583b62e6fcd71
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/13045
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/soc/intel/broadwell/acpi/gpio.asl

@@ -69,7 +69,7 @@ Device (GPIO)
 
 	// GWAK: Setup GPIO as ACPI GPE for Wake
 	// Arg0: GPIO Number
-	Method (GWAK, 1, NotSerialized)
+	Method (GWAK, 1, Serialized)
 	{
 		// Local0 = GPIO Base Address
 		Store (And (GPBS, Not(0x1)), Local0)