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_n5x_vab_defconfig b/configs/socfpga_n5x_vab_defconfig
index 82a4f98..8efdab8 100644
--- a/configs/socfpga_n5x_vab_defconfig
+++ b/configs/socfpga_n5x_vab_defconfig
@@ -29,6 +29,8 @@
 CONFIG_BOOTCOMMAND="run fatscript; run mmcfitload; run mmcfitboot"
 CONFIG_SPL_MAX_SIZE=0x40000
 CONFIG_SPL_BSS_MAX_SIZE=0x100000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xffe3f000
 CONFIG_SPL_CRC32=y
 CONFIG_SPL_CACHE=y
 CONFIG_SPL_SPI_LOAD=y