Kconfig: Move SPL settings into their own file

Move the SPL settings into common/spl where most of the SPL code is kept.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/Wits_Pro_A20_DKT_defconfig b/configs/Wits_Pro_A20_DKT_defconfig
index bfc8cba..0eaba02 100644
--- a/configs/Wits_Pro_A20_DKT_defconfig
+++ b/configs/Wits_Pro_A20_DKT_defconfig
@@ -10,8 +10,8 @@
 CONFIG_VIDEO_LCD_PANEL_LVDS=y
 CONFIG_DEFAULT_DEVICE_TREE="sun7i-a20-wits-pro-a20-dkt"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
-CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="SUNXI_GMAC,RGMII,AHCI"
+CONFIG_SPL=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_FPGA is not set