x86/acpigen: Fix ACPI _ROM method

Fix the following Error:
FAILED [LOW] AMLAsmASL_MSG_SERIALIZED_REQUIRED: Test 1, Assembler remark in line
142
Line | AML source
--------------------------------------------------------------------------------
00139|
00140|     Scope (\_SB.PCI0.IGFX)
00141|     {
00142|         Method (_ROM, 2, NotSerialized)  // _ROM: Read-Only Memory
     |                   ^
     | Remark 2120: Control Method should be made Serialized    (due to creation of named objects within)
00143|         {
00144|             OperationRegion (ROMS, SystemMemory, 0xCD520000, 0xFE00)
00145|             Field (ROMS, AnyAcc, NoLock, Preserve)
================================================================================

ADVICE: (for Remark #2120, ASL_MSG_SERIALIZED_REQUIRED): A named object is
created inside a non-serialized method - this method should be serialized. It is
possible that one thread enters the method and blocks and then a second thread
also executes the method, ending up in two attempts to create the object and
causing a failure.

Use the acpigen_write_method_serialized() to correct the error.

BUG=b:112476331
TEST=Run FWTS.

Change-Id: I145c3c3103efb4a02b4e02dd177f4bf50a2c7b3e
Signed-off-by: Marc Jones <marcj303@gmail.com>
Reviewed-on: https://review.coreboot.org/28124
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/arch/x86/acpigen.c

@@ -1454,7 +1454,7 @@ void acpigen_write_rom(void *bios, const size_t length)
 	ASSERT(length)
 
 	/* Method (_ROM, 2, NotSerialized) */
-	acpigen_write_method("_ROM", 2);
+	acpigen_write_method_serialized("_ROM", 2);
 
 	/* OperationRegion("ROMS", SYSTEMMEMORY, current, length) */
 	struct opregion opreg = OPREGION("ROMS", SYSTEMMEMORY,