boot: add disable_cache_rom() function

On certain architectures such as x86 the bootstrap processor
does most of the work. When CACHE_ROM is employed it's appropriate
to ensure that the caching enablement of the ROM is disabled so that
the caching settings are symmetric before booting the payload or OS.

Tested this on an x86 machine that turned on ROM caching. Linux did not
complain about asymmetric MTRR settings nor did the ROM show up as
cached in the MTRR settings.

Change-Id: Ia32ff9fdb1608667a0e9a5f23b9c8af27d589047
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2980
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/lib/selfboot.c

@@ -20,6 +20,7 @@
 
 #include <arch/byteorder.h>
 #include <console/console.h>
+#include <cpu/cpu.h>
 #include <fallback.h>
 #include <boot/elf.h>
 #include <boot/elf_boot.h>
@@ -540,6 +541,10 @@ int selfboot(struct lb_memory *mem, struct cbfs_payload *payload)
 	coverage_exit();
 #endif
 
+	/* Tear down the caching of the ROM. */
+	if (disable_cache_rom)
+		disable_cache_rom();
+
 	/* Before we go off to run the payload, see if
 	 * we stayed within our bounds.
 	 */