cpu/intel: Refactor platform_enter_postcar()

There are benefits in placing the postcar_frame structure
in .bss and returning control to romstage_main().

Change-Id: I0418a2abc74f749203c587b2763c5f8a5960e4f9
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34808
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/cpu/intel/romstage.h

@@ -5,6 +5,10 @@
 
 void mainboard_romstage_entry(unsigned long bist);
 
-void platform_enter_postcar(void);
+/* fill_postcar_frame() is called after raminit completes and right before
+ * calling run_postcar_phase(). Implementation should call postcar_frame_add_mtrr()
+ * to tag memory ranges as cacheable to speed up execution of postcar and
+ * early ramstage. */
+void fill_postcar_frame(struct postcar_frame *pcf);
 
 #endif /* _CPU_INTEL_ROMSTAGE_H */