lib: Make log2() available in romstage on ARM, not just x86

On x86, log2() is defined as an inline function in arch/io.h. This is
a remnant of ROMCC, and forced us to not include clog2.c in romstage.
As a result, romstage on ARM has no log2().
Use the inline log2 only with ROMCC, but otherwise, use the one in
clog2.c.

Change-Id: Ifef2aa0a7b5a1db071a66f2eec0be421b8b2a56d
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/4681
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/northbridge/intel/i945/raminit.c

@@ -24,6 +24,7 @@
 #include <spd.h>
 #include <string.h>
 #include <arch/io.h>
+#include <lib.h>
 #include "raminit.h"
 #include "i945.h"
 #include <cbmem.h>