ddr: altera: soc64: Integer fix overflow that caused DDR size mismatched

Convert the constant integer to 'phys_size_t' to avoid overflow
when calculating the SDRAM size.

Signed-off-by: Dinesh Maniyam <dinesh.maniyam@intel.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@intel.com>