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/xilinx_zynqmp_virt_defconfig b/configs/xilinx_zynqmp_virt_defconfig
index 40c9c28..88d5fec 100644
--- a/configs/xilinx_zynqmp_virt_defconfig
+++ b/configs/xilinx_zynqmp_virt_defconfig
@@ -33,6 +33,8 @@
 CONFIG_CLOCKS=y
 CONFIG_SPL_MAX_SIZE=0x40000
 CONFIG_SPL_BSS_MAX_SIZE=0x80000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xfffffffc
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_FS_LOAD_KERNEL_NAME="atf-uboot.ub"
 CONFIG_SPL_FS_LOAD_ARGS_NAME="u-boot.bin"