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>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 584f848..0711cbf 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -500,7 +500,7 @@
 	  future requests for that data can be served faster. Enable this option
 	  to build the drivers in drivers/cache as part of an SPL build.
 
-config SPL_CPU_SUPPORT
+config SPL_CPU
 	bool "Support CPU drivers"
 	help
 	  Enable this to support CPU drivers in SPL. These drivers can set