soc/intel/quark: Return NULL for top_of_memory

Return NULL for top_of_memory when the register has not been set.

TEST=Build and run on Galileo Gen2

Change-Id: If79cac68c2a64aa9bf3be72d3cfc4c73fceef12b
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/18802
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: Martin Roth <martinroth@google.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/intel/quark/memmap.c

@@ -26,7 +26,8 @@ void *cbmem_top(void)
 	top_of_memory <<= 16;
 
 	/* Reserve 64 KiB for RMU firmware */
-	top_of_memory -= 0x10000;
+	if (top_of_memory)
+		top_of_memory -= 0x10000;
 
 	/* Return the top of memory */
 	return (void *)top_of_memory;