configs: socfpga: Add CONFIG_SPL_PAD_TO

Add CONFIG_SPL_PAD_TO for Gen5 and Arria 10. CONFIG_SPL_PAD_TO is set to
size of OCRAM.

This is preparation for image padding change in socfpgaimage.

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>