change AXI4RAM sim size to 512MB to sync with sim device tree; avoid …

…alias problem

src/main/scala/sim/NutShellSim.scala

@@ -38,7 +38,7 @@ class SimTop extends Module {
 
   lazy val config = NutCoreConfig(FPGAPlatform = false)
   val soc = Module(new NutShell()(config))
-  val mem = Module(new AXI4RAM(memByte = 128 * 1024 * 1024, useBlackBox = true))
+  val mem = Module(new AXI4RAM(memByte = 512 * 1024 * 1024, useBlackBox = true))
   // Be careful with the commit checking of emu.
   // A large delay will make emu incorrectly report getting stuck.
   val memdelay = Module(new AXI4Delayer(0))