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/q8_a23_tablet_800x480_defconfig b/configs/q8_a23_tablet_800x480_defconfig
index 6053587..11d208a 100644
--- a/configs/q8_a23_tablet_800x480_defconfig
+++ b/configs/q8_a23_tablet_800x480_defconfig
@@ -16,6 +16,7 @@
 CONFIG_VIDEO_LCD_BL_EN="PH6"
 CONFIG_VIDEO_LCD_BL_PWM="PH0"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_AXP_DLDO1_VOLT=3300
 CONFIG_CONS_INDEX=5