arm: socfpga: Move Stratix10 and Agilex SPL common code

Move Stratix10 and Agilex SPL common code to spl_soc64.c.
We are in preparation for new n5x device support.
No functional change in this patch.

Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>