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/pine_h64_defconfig b/configs/pine_h64_defconfig
index 7466c3b..2cddcf5 100644
--- a/configs/pine_h64_defconfig
+++ b/configs/pine_h64_defconfig
@@ -11,6 +11,7 @@
 CONFIG_SPL_SPI_SUNXI=y
 # CONFIG_PSCI_RESET is not set
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x118000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SUPPORT_EMMC_BOOT=y
 CONFIG_SPI_FLASH_WINBOND=y