mb/qemu-{i440fx,q35}: Use POSTCAR stage to load the ramstage

Qemu does not have a real CAR but postcar stage is still useful
for testing the stage.

The postcar stage is also mandatory for x86_64 to setup
pagetables for x86_64 ramstage.

Do not set up MTRRs, as qemu ignores them anyways.

Tested on qemu-i440fx and qemu-q35.

Change-Id: I6638534d99fde312e55b6a6be8c95e4cb25cca80
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/30499
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/mainboard/emulation/qemu-i440fx/Kconfig

@@ -12,7 +12,8 @@ config BOARD_SPECIFIC_OPTIONS
 	select BOARD_ROMSIZE_KB_256
 	select MAINBOARD_HAS_NATIVE_VGA_INIT
 	select MAINBOARD_FORCE_NATIVE_VGA_INIT
-	select NO_CAR_GLOBAL_MIGRATION
+	select POSTCAR_STAGE
+	select POSTCAR_CONSOLE
 
 config MAINBOARD_DIR
 	string