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/tbs_a711_defconfig b/configs/tbs_a711_defconfig
index a8a9b85..3dd9252 100644
--- a/configs/tbs_a711_defconfig
+++ b/configs/tbs_a711_defconfig
@@ -13,6 +13,7 @@
 CONFIG_USB0_ID_DET="PH11"
 CONFIG_AXP_GPIO=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_FASTBOOT_CMD_OEM_FORMAT=y
 CONFIG_AXP_DCDC5_VOLT=1200