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/orangepi_zero_plus2_defconfig b/configs/orangepi_zero_plus2_defconfig
index 623dd04..7904012 100644
--- a/configs/orangepi_zero_plus2_defconfig
+++ b/configs/orangepi_zero_plus2_defconfig
@@ -9,6 +9,7 @@
 CONFIG_MMC0_CD_PIN="PH13"
 CONFIG_MMC_SUNXI_SLOT_EXTRA=2
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x54000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SUN8I_EMAC=y
 CONFIG_USB_EHCI_HCD=y