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/evb-rk3308_defconfig b/configs/evb-rk3308_defconfig
index 8d9ca36..e871ba1 100644
--- a/configs/evb-rk3308_defconfig
+++ b/configs/evb-rk3308_defconfig
@@ -27,6 +27,8 @@
 CONFIG_SPL_MAX_SIZE=0x20000
 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_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set