board: imx6dl_mamoj: set the value for CSF size

Default value is too high and it covers the worst case. SPL for
imx6dl_mamoj becomes too big and cannot fit into the OCRAM, but CSF size
can be reduced.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Jagan Teki <jagan@amarulasolutions.com>
CC: Breno Lima <breno.lima@nxp.com>