drivers/ddr/fsl: Update fsl_ddr_board_options as weak function

fsl_ddr_board_options is generally defined in board
board's ddr.c, but some boards like lx2160ardb board
does not need this function.
Defining fsl_ddr_board_options as weak function to
resolve compilation errors for such boards.

Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
[PK: Fix checkpatch warnings]
Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>