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/Sunchip_CX-A99_defconfig b/configs/Sunchip_CX-A99_defconfig
index ee0c15b..749bf1c 100644
--- a/configs/Sunchip_CX-A99_defconfig
+++ b/configs/Sunchip_CX-A99_defconfig
@@ -12,4 +12,5 @@
 CONFIG_USB1_VBUS_PIN="PL7"
 CONFIG_USB3_VBUS_PIN="PL8"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x18000
 CONFIG_SYS_PBSIZE=1024