src/cpu: 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: I4e5e585c3f98a129d89ef38b26d828d3bfeac7cf
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20356
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/cpu/x86/smm/smihandler.c

@@ -19,7 +19,7 @@
 #include <cpu/x86/cache.h>
 #include <cpu/x86/smm.h>
 
-#if CONFIG_SPI_FLASH_SMM
+#if IS_ENABLED(CONFIG_SPI_FLASH_SMM)
 #include <spi-generic.h>
 #endif
 
@@ -185,7 +185,7 @@ void smi_handler(u32 smm_revision)
 
 	/* Allow drivers to initialize variables in SMM context. */
 	if (do_driver_init) {
-#if CONFIG_SPI_FLASH_SMM
+#if IS_ENABLED(CONFIG_SPI_FLASH_SMM)
 		spi_init();
 #endif
 		do_driver_init = 0;