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/fu540/Kconfig b/arch/riscv/cpu/fu540/Kconfig
index 61bd5c4..616b256 100644
--- a/arch/riscv/cpu/fu540/Kconfig
+++ b/arch/riscv/cpu/fu540/Kconfig
@@ -13,7 +13,7 @@
 	imply RISCV_TIMER if (RISCV_SMODE || SPL_RISCV_SMODE)
 	imply SIFIVE_CLINT if (RISCV_MMODE || SPL_RISCV_MMODE)
 	imply CMD_CPU
-	imply SPL_CPU_SUPPORT
+	imply SPL_CPU
 	imply SPL_OPENSBI
 	imply SPL_LOAD_FIT
 	imply SMP