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_a13_tablet_defconfig b/configs/q8_a13_tablet_defconfig
index 08be881..83981d3 100644
--- a/configs/q8_a13_tablet_defconfig
+++ b/configs/q8_a13_tablet_defconfig
@@ -15,6 +15,7 @@
 CONFIG_VIDEO_LCD_BL_EN="AXP0-1"
 CONFIG_VIDEO_LCD_BL_PWM="PB2"
 # 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