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/pinetab_defconfig b/configs/pinetab_defconfig
index 4ecd241..f90cb0d 100644
--- a/configs/pinetab_defconfig
+++ b/configs/pinetab_defconfig
@@ -8,4 +8,5 @@
 CONFIG_DRAM_ZQ=3881949
 CONFIG_MMC_SUNXI_SLOT_EXTRA=2
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x54000
 CONFIG_SYS_PBSIZE=1024