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/nanopi_m1_defconfig b/configs/nanopi_m1_defconfig
index 547c203..47a6b78 100644
--- a/configs/nanopi_m1_defconfig
+++ b/configs/nanopi_m1_defconfig
@@ -5,6 +5,7 @@
 CONFIG_MACH_SUN8I_H3=y
 CONFIG_DRAM_CLK=408
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_OHCI_HCD=y