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/socfpga_secu1_defconfig b/configs/socfpga_secu1_defconfig
index d59e841..72cf0d1 100644
--- a/configs/socfpga_secu1_defconfig
+++ b/configs/socfpga_secu1_defconfig
@@ -36,6 +36,8 @@
 CONFIG_SPL_PAD_TO=0x10000
 CONFIG_SPL_NO_BSS_LIMIT=y
 CONFIG_SPL_LEGACY_IMAGE_CRC_CHECK=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x0
 # CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR is not set
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION=0x1
 # CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_PARTITION_TYPE is not set