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/configs/qemu-x86_64_defconfig b/configs/qemu-x86_64_defconfig
index 99a489c..4815d8a 100644
--- a/configs/qemu-x86_64_defconfig
+++ b/configs/qemu-x86_64_defconfig
@@ -31,7 +31,7 @@
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_PCI_INIT_R=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
-CONFIG_SPL_CPU_SUPPORT=y
+CONFIG_SPL_CPU=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_NET_SUPPORT=y