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/ls1046ardb_qspi_spl_defconfig b/configs/ls1046ardb_qspi_spl_defconfig
index 39e38e4..ae7d0b3 100644
--- a/configs/ls1046ardb_qspi_spl_defconfig
+++ b/configs/ls1046ardb_qspi_spl_defconfig
@@ -46,6 +46,8 @@
 CONFIG_SPL_PAD_TO=0x20000
 CONFIG_SPL_BSS_MAX_SIZE=0x80000
 CONFIG_SPL_BOARD_INIT=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x10020000
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y