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/MK808C_defconfig b/configs/MK808C_defconfig
index f4d31b3..4e678bd 100644
--- a/configs/MK808C_defconfig
+++ b/configs/MK808C_defconfig
@@ -5,6 +5,7 @@
 CONFIG_MACH_SUN7I=y
 CONFIG_DRAM_CLK=384
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SYS_I2C_MVTWSI=y