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_atf_defconfig b/configs/socfpga_n5x_atf_defconfig
index 7c7f901..88763d1 100644
--- a/configs/socfpga_n5x_atf_defconfig
+++ b/configs/socfpga_n5x_atf_defconfig
@@ -28,6 +28,8 @@
 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