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/sun8i_a23_evb_defconfig b/configs/sun8i_a23_evb_defconfig
index 21ea2ba..59315cd 100644
--- a/configs/sun8i_a23_evb_defconfig
+++ b/configs/sun8i_a23_evb_defconfig
@@ -9,6 +9,7 @@
 CONFIG_USB0_VBUS_DET="axp_vbus_detect"
 CONFIG_USB1_VBUS_PIN="PH7"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_CONS_INDEX=5
 CONFIG_USB_EHCI_HCD=y