cpu: Rename SPL_CPU_SUPPORT to SPL_CPU

The _SUPPORT suffix is from an earlier time and interferes with use of
the CONFIG_IS_ENABLED() macro. Rename the option to drop the suffix.

Tidy up the TODO that prompted this.

Signed-off-by: Simon Glass <sjg@chromium.org>

drivers/timer/timer-uclass.c

@@ -83,11 +83,7 @@ static int timer_post_probe(struct udevice *dev)
 	return 0;
 }
 
-/*
- * TODO: should be CONFIG_IS_ENABLED(CPU), but the SPL config has _SUPPORT on
- * the end...
- */
-#if defined(CONFIG_CPU) || defined(CONFIG_SPL_CPU_SUPPORT)
+#if CONFIG_IS_ENABLED(CPU)
 int timer_timebase_fallback(struct udevice *dev)
 {
 	struct udevice *cpu;