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/giedi_defconfig b/configs/giedi_defconfig
index ae05c9a..ec6082a 100644
--- a/configs/giedi_defconfig
+++ b/configs/giedi_defconfig
@@ -40,6 +40,8 @@
 CONFIG_SPL_BSS_MAX_SIZE=0x1000
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x13e000
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x800
 CONFIG_SPL_POWER_DOMAIN=y