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/CHIP_pro_defconfig b/configs/CHIP_pro_defconfig
index 16456c1..9016801 100644
--- a/configs/CHIP_pro_defconfig
+++ b/configs/CHIP_pro_defconfig
@@ -5,6 +5,7 @@
 CONFIG_MACH_SUN5I=y
 CONFIG_DRAM_TIMINGS_DDR3_800E_1066G_1333J=y
 CONFIG_USB0_VBUS_PIN="PB10"
+CONFIG_SPL_STACK=0x8000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_PBSIZE=1024
 # CONFIG_CMD_FLASH is not set