cpu/intel: add IS_ENABLED() around Kconfig symbol references

Some of these can be changed from #if to if(), but that will happen
in a follow-on commmit.

Change-Id: Ie685bbbb1cbf06d32631ea40ad120b6f45374b2e
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20336
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/cpu/intel/model_206ax/bootblock.c

@@ -24,7 +24,8 @@
 #include <cpu/intel/microcode/microcode.c>
 #include "model_206ax.h"
 
-#if CONFIG_SOUTHBRIDGE_INTEL_BD82X6X || CONFIG_SOUTHBRIDGE_INTEL_C216
+#if IS_ENABLED(CONFIG_SOUTHBRIDGE_INTEL_BD82X6X) || \
+	IS_ENABLED(CONFIG_SOUTHBRIDGE_INTEL_C216)
 /* Needed for RCBA access to set Soft Reset Data register */
 #include <southbridge/intel/bd82x6x/pch.h>
 #else