cpu/amd: 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: I9f4155285529ec28e826637a61436478f648704c
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20335
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/cpu/amd/geode_lx/syspreinit.c

@@ -32,7 +32,7 @@ static void StartTimer1(void)
 void SystemPreInit(void)
 {
 	/* they want a jump ... */
-#if !CONFIG_CACHE_AS_RAM
+#if !IS_ENABLED(CONFIG_CACHE_AS_RAM)
 	__asm__ __volatile__("jmp .+2\ninvd\njmp .+2\n");
 #endif
 	StartTimer1();