imx6: SPL support for iMX6 SabreSD

This patch will enable the support for SPL on iMX6 SabreSD.
It tested on SD2 and SD3 mmc port (Switch 1 or 2 of SW6)

Signed-off-by: John Tobias <john.tobias.ph@gmail.com>
Acked-by: Stefano Babic <sbabic@denx.de>