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_de1_soc_defconfig b/configs/socfpga_de1_soc_defconfig
index b800f27..749ec54 100644
--- a/configs/socfpga_de1_soc_defconfig
+++ b/configs/socfpga_de1_soc_defconfig
@@ -23,6 +23,8 @@
 CONFIG_SPL_PAD_TO=0x10000
 CONFIG_SPL_NO_BSS_LIMIT=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x0
 CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_SYS_MAXARGS=32
 CONFIG_CMD_ASKENV=y