soc/intel/skylake: 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.

Change-Id: Ice217561bb01a43ba520ce51e03d81979f317343
Signed-off-by: Barnali Sarkar <barnali.sarkar@intel.com>
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/20089
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/intel/skylake/cpu.c

@@ -34,6 +34,7 @@
 #include <cpu/x86/name.h>
 #include <cpu/x86/smm.h>
 #include <delay.h>
+#include <intelblocks/fast_spi.h>
 #include <pc80/mc146818rtc.h>
 #include <soc/cpu.h>
 #include <soc/msr.h>
@@ -569,6 +570,12 @@ static void soc_post_cpus_init(void *unused)
 {
 	if (mp_run_on_all_cpus(&x86_setup_mtrrs_with_detect, 1000) < 0)
 		printk(BIOS_ERR, "MTRR programming failure\n");
+
+	/* Temporarily cache the memory-mapped boot media. */
+	if (IS_ENABLED(CONFIG_BOOT_DEVICE_MEMORY_MAPPED) &&
+		IS_ENABLED(CONFIG_BOOT_DEVICE_SPI_FLASH))
+		fast_spi_cache_bios_region();
+
 	x86_mtrr_check();
 }