[SANTINO] Add SPL_TEXT_BASE to seco_imx6dl_santino_defconfig

Configure the SPL base to let `uuu` tool execut the SPD.

This commit shows the first output through board's serial port.

configs/seco_imx6dl_santino_defconfig

@@ -5,6 +5,7 @@ CONFIG_MX6DL=y
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_TARGET_MX6DL_SECO_SANTINO=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg,MX6DL,SPL"
+CONFIG_SPL_TEXT_BASE=0x00908000
 CONFIG_ENV_OFFSET=0xE0000
 CONFIG_ENV_SIZE=0x2000