Convert CONFIG_SPL_STACK to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_STACK

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/microblaze/cpu/start.S b/arch/microblaze/cpu/start.S
index 645f7cb..25e9968 100644
--- a/arch/microblaze/cpu/start.S
+++ b/arch/microblaze/cpu/start.S
@@ -19,7 +19,7 @@
 	mts	rslr, r8
 
 #if defined(CONFIG_SPL_BUILD)
-	addi	r1, r0, CONFIG_SPL_STACK_ADDR
+	addi	r1, r0, CONFIG_SPL_STACK
 #else
 	addi	r1, r0, CONFIG_SYS_INIT_SP_OFFSET
 #endif