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_mini_emmc0_defconfig b/configs/xilinx_zynqmp_mini_emmc0_defconfig
index 058e164..fe8de73 100644
--- a/configs/xilinx_zynqmp_mini_emmc0_defconfig
+++ b/configs/xilinx_zynqmp_mini_emmc0_defconfig
@@ -26,6 +26,8 @@
 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_CMDLINE_EDITING is not set
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_SYS_MAXARGS=64