Pass sdrc timing values through board_sdrc_timings structure

Pass sdrc timing values through board_sdrc_timings structure

Instead of passing individual registers by value to board_get_mem_timings,
pass a board_mem_timings structure pointer for the board files to fill in.
Pass same structure pointer to write_sdrc_timings.  This saves about
90 bytes of space in SPL.

Signed-off-by: Peter Barada <peter.barada@logicpd.com>