ddr: socfpga: gen5: constify altera_gen5_sdram_ops

Make the function pointer struct const, as it does not need to be
writable. This doesn't really change anything other than moving this
variable to a different section. No functional change.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>