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/zynq_cse_nand_defconfig b/configs/zynq_cse_nand_defconfig
index cb71813..f63a2bb 100644
--- a/configs/zynq_cse_nand_defconfig
+++ b/configs/zynq_cse_nand_defconfig
@@ -23,6 +23,8 @@
 CONFIG_CLOCKS=y
 CONFIG_SPL_MAX_SIZE=0x30000
 CONFIG_SPL_BSS_MAX_SIZE=0x8000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xfffffe00
 CONFIG_SPL_STACK_R=y
 # CONFIG_CMDLINE_EDITING is not set
 # CONFIG_AUTO_COMPLETE is not set