soc/intel/apollolake: cache the MMIO BIOS region

If the boot media is memory mapped temporarily mark it as write
protect MTRR type so that memory-mapped accesses are faster.

Depthcharge payload loading was sped up by 75ms using this.

BUG=chrome-os-partner:56656,chrome-os-partner:59682

Change-Id: Iba87a51a05559d81b8e00fa4f6824dacf7a661f5
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/17372
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Andrey Petrov <andrey.petrov@intel.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/soc/intel/apollolake/cpu.c

@@ -183,4 +183,9 @@ void apollolake_init_cpus(device_t dev)
 	/* Clear for take-off */
 	if (mp_init_with_smm(dev->link_list, &mp_ops) < 0)
 		printk(BIOS_ERR, "MP initialization failure.\n");
+
+	/* Temporarily cache the memory-mapped boot media. */
+	if (IS_ENABLED(CONFIG_BOOT_DEVICE_MEMORY_MAPPED))
+		mtrr_use_temp_range(-CONFIG_ROM_SIZE, CONFIG_ROM_SIZE,
+					MTRR_TYPE_WRPROT);
 }