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/tanix_tx6_defconfig b/configs/tanix_tx6_defconfig
index cf34b50..d1f12fb 100644
--- a/configs/tanix_tx6_defconfig
+++ b/configs/tanix_tx6_defconfig
@@ -8,4 +8,5 @@
 CONFIG_MMC0_CD_PIN="PF6"
 CONFIG_MMC_SUNXI_SLOT_EXTRA=2
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x118000
 CONFIG_SYS_PBSIZE=1024