configs: mx6sabresd: Reduce SPL size by disabling DOS, EXT and EFI support

configs: mx6sabresd: Reduce SPL size by disabling DOS, EXT and EFI support

With DM and FIT enabled in SPL, there is an sram overflow. By disabling
CONFIG_SPL_DOS_PARTITION, CONFIG_SPL_EXT_SUPPORT and
CONFIG_SPL_EFI_PARTITION, we get to keep the 'one binary to fit all'
for imx6[q|qp|dl] on sabresd since the final SPL image is now under 64KB.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>