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/drivers/Makefile b/drivers/Makefile
index c562a71..3510dab 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -38,7 +38,7 @@
 
 obj-$(CONFIG_SPL_BOOTCOUNT_LIMIT) += bootcount/
 obj-$(CONFIG_SPL_CACHE_SUPPORT) += cache/
-obj-$(CONFIG_SPL_CPU_SUPPORT) += cpu/
+obj-$(CONFIG_SPL_CPU) += cpu/
 obj-$(CONFIG_SPL_CRYPTO_SUPPORT) += crypto/
 obj-$(CONFIG_SPL_MPC8XXX_INIT_DDR_SUPPORT) += ddr/fsl/
 obj-$(CONFIG_ARMADA_38X) += ddr/marvell/a38x/