ddr: altera: Restructure Stratix 10 SDRAM driver

Restructure Stratix 10 SDRAM driver. Move common code to separate
file, in preparation to support SDRAM driver for Agilex.

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
diff --git a/drivers/ddr/altera/Makefile b/drivers/ddr/altera/Makefile
index 341ac0d..eb8da13b 100644
--- a/drivers/ddr/altera/Makefile
+++ b/drivers/ddr/altera/Makefile
@@ -9,5 +9,5 @@
 ifdef CONFIG_$(SPL_)ALTERA_SDRAM
 obj-$(CONFIG_TARGET_SOCFPGA_GEN5) += sdram_gen5.o sequencer.o
 obj-$(CONFIG_TARGET_SOCFPGA_ARRIA10) += sdram_arria10.o
-obj-$(CONFIG_TARGET_SOCFPGA_STRATIX10) += sdram_s10.o
+obj-$(CONFIG_TARGET_SOCFPGA_STRATIX10) += sdram_soc64.o sdram_s10.o
 endif