SDRC: prevent null pointer dereference if sdrc_init_params is null

omap2_sdrc_get_params() should check to see if a board-*.c file has
called omap2_init_common_hw() with a null pointer for the
omap_sdrc_params argument, rather than attempting to dereference it.
Otherwise, boot will fail after the "Reprogramming SDRC" boot message.
Problem found by Peter Barada <peterb@logicpd.com>.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Peter Barada <peterb@logicpd.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>