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/da850evm_defconfig b/configs/da850evm_defconfig
index 8ddde6a..a4bc7e4 100644
--- a/configs/da850evm_defconfig
+++ b/configs/da850evm_defconfig
@@ -39,6 +39,8 @@
 CONFIG_SPL_FOOTPRINT_LIMIT=y
 CONFIG_SPL_MAX_FOOTPRINT=0x8000
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x8001ff00
 # CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR is not set
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_SPI_LOAD=y