crossings: deprecate non-island crossing style

src/main/scala/amba/axi4/AsyncCrossing.scala

@@ -62,6 +62,7 @@ object AXI4AsyncCrossingSink
   }
 }
 
+@deprecated("AXI4AsyncCrossing is fragile. Use AXI4AsyncCrossingSource and AXI4AsyncCrossingSink", "rocket-chip 1.2")
 class AXI4AsyncCrossing(depth: Int = 8, sync: Int = 3)(implicit p: Parameters) extends LazyModule
 {
   val source = LazyModule(new AXI4AsyncCrossingSource(sync))

src/main/scala/tilelink/AsyncCrossing.scala

@@ -86,6 +86,7 @@ object TLAsyncCrossingSink
   }
 }
 
+@deprecated("TLAsyncCrossing is fragile. Use TLAsyncCrossingSource and TLAsyncCrossingSink", "rocket-chip 1.2")
 class TLAsyncCrossing(depth: Int = 8, sync: Int = 3)(implicit p: Parameters) extends LazyModule
 {
   val source = LazyModule(new TLAsyncCrossingSource(sync))

src/main/scala/tilelink/RationalCrossing.scala

@@ -97,6 +97,7 @@ object TLRationalCrossingSink
   }
 }
 
+@deprecated("TLRationalCrossing is fragile. Use TLRationalCrossingSource and TLRationalCrossingSink", "rocket-chip 1.2")
 class TLRationalCrossing(direction: RationalDirection = Symmetric)(implicit p: Parameters) extends LazyModule
 {
   val source = LazyModule(new TLRationalCrossingSource)