spl: Use CONFIG_VAL() to obtain the SPL stack

Now that we have the same option for SPL and TPL, simplify the logic for
determining the initial stack.

Note that this changes behaviour as current SPL_STACK is a fallback for
TPL. However, that was likely unintended and can be handled with Kconfig
defaults if needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Suggested-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/Kconfig.tpl b/common/spl/Kconfig.tpl
index 421352f..a535b61 100644
--- a/common/spl/Kconfig.tpl
+++ b/common/spl/Kconfig.tpl
@@ -138,8 +138,8 @@
 	bool "TPL requires a initial, fixed, stack-pointer location"
 	help
 	  Enable if the TPL phase should not inherit its initial
-	  stack-pointer from the settings for U-Boot proper (or SPL if
-	  SPL_STACK is defined), but should set its own value.
+	  stack-pointer from the settings for U-Boot proper, but should set its
+	  own value.
 
 config TPL_STACK
 	hex "Address of the initial stack-pointer for the TPL phase"