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/arch/riscv/cpu/ax25/Kconfig b/arch/riscv/cpu/ax25/Kconfig
index 327b74e..941d963 100644
--- a/arch/riscv/cpu/ax25/Kconfig
+++ b/arch/riscv/cpu/ax25/Kconfig
@@ -6,7 +6,7 @@
 	imply RISCV_TIMER if (RISCV_SMODE || SPL_RISCV_SMODE)
 	imply ANDES_PLIC if (RISCV_MMODE || SPL_RISCV_MMODE)
 	imply ANDES_PLMT_TIMER if (RISCV_MMODE || SPL_RISCV_MMODE)
-	imply SPL_CPU_SUPPORT
+	imply SPL_CPU
 	imply SPL_OPENSBI
 	imply SPL_LOAD_FIT
 	help