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/mixtile_loftq_defconfig b/configs/mixtile_loftq_defconfig
index 0cd94d7..11e3dfc 100644
--- a/configs/mixtile_loftq_defconfig
+++ b/configs/mixtile_loftq_defconfig
@@ -9,6 +9,7 @@
 CONFIG_USB1_VBUS_PIN="PH24"
 CONFIG_USB2_VBUS_PIN=""
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_PHY_REALTEK=y
 CONFIG_ETH_DESIGNWARE=y