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/silinux_ek874_defconfig b/configs/silinux_ek874_defconfig
index e728cb2..ce211a9 100644
--- a/configs/silinux_ek874_defconfig
+++ b/configs/silinux_ek874_defconfig
@@ -25,6 +25,8 @@
 # CONFIG_BOARD_EARLY_INIT_F is not set
 CONFIG_SYS_MALLOC_BOOTPARAMS=y
 CONFIG_SPL_BSS_MAX_SIZE=0x1000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xe6304000
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=2068