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/ls2080aqds_nand_defconfig b/configs/ls2080aqds_nand_defconfig
index 0c57920..08f9536 100644
--- a/configs/ls2080aqds_nand_defconfig
+++ b/configs/ls2080aqds_nand_defconfig
@@ -36,6 +36,8 @@
 CONFIG_SPL_MAX_SIZE=0x16000
 CONFIG_SPL_PAD_TO=0x20000
 CONFIG_SPL_BSS_MAX_SIZE=0x100000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x18009ff0
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y