arch/x86: Make postcar TempRamExit call generic

Move the FSP-specific call for tearing down cache-as-RAM out of
postcar.c and replace it with an empty weak function.

This patch omits checking if (IS_ENABLED(CONFIG_FSP_CAR)).  The
temp_ram_exit.c file with the real fsp_temp_ram_exit() is only built
when CONFIG_FSP_CAR is true.

Change-Id: I9adbb1f2a7b2ff50d9f36d5a3640f63410c09479
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/20965
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/intel/fsp2_0/temp_ram_exit.c

@@ -47,3 +47,8 @@ void fsp_temp_ram_exit(void)
 		die("TempRamExit returned an error!\n");
 	}
 }
+
+void late_car_teardown(void)
+{
+	fsp_temp_ram_exit();
+}