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/drivers/ddr/altera/Kconfig b/drivers/ddr/altera/Kconfig
index 8f60b56..83c1ab5 100644
--- a/drivers/ddr/altera/Kconfig
+++ b/drivers/ddr/altera/Kconfig
@@ -1,5 +1,6 @@
-config ALTERA_SDRAM
-	bool "SoCFPGA DDR SDRAM driver"
+config SPL_ALTERA_SDRAM
+	bool "SoCFPGA DDR SDRAM driver in SPL"
+	depends on SPL
 	depends on TARGET_SOCFPGA_GEN5 || TARGET_SOCFPGA_ARRIA10
 	select RAM if TARGET_SOCFPGA_GEN5
 	select SPL_RAM if TARGET_SOCFPGA_GEN5