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/chromebook_coral_defconfig b/configs/chromebook_coral_defconfig
index e3756cf..4823913 100644
--- a/configs/chromebook_coral_defconfig
+++ b/configs/chromebook_coral_defconfig
@@ -43,7 +43,7 @@
 CONFIG_HANDOFF=y
 CONFIG_TPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_SEPARATE_BSS=y
-CONFIG_SPL_CPU_SUPPORT=y
+CONFIG_SPL_CPU=y
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_PCI=y
 # CONFIG_SPL_SPI_FLASH_TINY is not set