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/bananapi_m2_zero_defconfig b/configs/bananapi_m2_zero_defconfig
index 74c164c..6a3594c 100644
--- a/configs/bananapi_m2_zero_defconfig
+++ b/configs/bananapi_m2_zero_defconfig
@@ -6,4 +6,5 @@
 CONFIG_DRAM_CLK=408
 CONFIG_MMC0_CD_PIN=""
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024