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/ficus-rk3399_defconfig b/configs/ficus-rk3399_defconfig
index fac8889..7012c02 100644
--- a/configs/ficus-rk3399_defconfig
+++ b/configs/ficus-rk3399_defconfig
@@ -20,6 +20,8 @@
 CONFIG_SPL_PAD_TO=0x7f8000
 CONFIG_SPL_BSS_MAX_SIZE=0x10000
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xff8effff
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x4000
 CONFIG_CMD_BOOTZ=y