Add ddr3 memory support

Currently DTG doesn't support ddr3 as memory, so ddr3 will
be generated as peripheral in pl.dtsi and the memory node is
empty. Hence add support for ddr3 as memory.

Signed-off-by: Venkatesh Yadav Abbarapu <vabbarap@xilinx.com>

mig_7series/data/mig_7series.mdd

@@ -16,7 +16,7 @@ OPTION psf_version = 3.0;
 
 BEGIN driver mig_7series
 
-  OPTION supported_peripherals = (mig_7series ddr4);
+  OPTION supported_peripherals = (mig_7series ddr4 ddr3);
   OPTION supported_os_types = (DTS);
   OPTION driver_state = ACTIVE;
   OPTION NAME = mig_7series;