ddr: altera: Compile ALTERA SDRAM in SPL only

Compile ALTERA_SDRAM driver in SPL only.
Rename ALTERA_SDRAM to SPL_ALTERA_SDRAM.

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
diff --git a/Makefile b/Makefile
index d6a6ef1..5ae5db5 100644
--- a/Makefile
+++ b/Makefile
@@ -713,7 +713,7 @@
 libs-$(CONFIG_FMAN_ENET) += drivers/net/fm/
 libs-$(CONFIG_SYS_FSL_DDR) += drivers/ddr/fsl/
 libs-$(CONFIG_SYS_FSL_MMDC) += drivers/ddr/fsl/
-libs-$(CONFIG_ALTERA_SDRAM) += drivers/ddr/altera/
+libs-$(CONFIG_$(SPL_)ALTERA_SDRAM) += drivers/ddr/altera/
 libs-y += drivers/serial/
 libs-y += drivers/usb/dwc3/
 libs-y += drivers/usb/common/