imx:mx6sxsabresd add spl config file

Add a SPL default configuration file for mx6sxsabresd board.
Signed-off-by: NPeng Fan <Peng.Fan@freescale.com>

configs/mx6sxsabresd_spl_defconfig

+CONFIG_SPL=y
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6SX"
++S:CONFIG_ARM=y
++S:CONFIG_TARGET_MX6SXSABRESD=y