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/configs/leez-rk3399_defconfig b/configs/leez-rk3399_defconfig
index c8e4628..d524e45 100644
--- a/configs/leez-rk3399_defconfig
+++ b/configs/leez-rk3399_defconfig
@@ -19,6 +19,8 @@
 CONFIG_SPL_MAX_SIZE=0x2e000
 CONFIG_SPL_PAD_TO=0x7f8000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x400000
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x10000
 CONFIG_TPL=y