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/db-88f6720_defconfig b/configs/db-88f6720_defconfig
index cf7bd17..77e0bbc 100644
--- a/configs/db-88f6720_defconfig
+++ b/configs/db-88f6720_defconfig
@@ -28,6 +28,8 @@
 CONFIG_SPL_MAX_SIZE=0x1ffd0
 CONFIG_SPL_BSS_MAX_SIZE=0x4000
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x4002c000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_MAXARGS=32
 CONFIG_CMD_I2C=y