PLIC: Use longer DTS name for Max Priorities.

I used the singular because there is really only one max priority

src/main/scala/devices/tilelink/Plic.scala

@@ -68,7 +68,7 @@ class TLPLIC(params: PLICParams)(implicit p: Parameters) extends LazyModule
       val extra = Map(
         "interrupt-controller" -> Nil,
         "riscv,ndev" -> Seq(ResourceInt(nDevices)),
-        "riscv,maxpri" -> Seq(ResourceInt(maxPriorities)),        
+        "riscv,max-priority" -> Seq(ResourceInt(maxPriorities)),        
         "#interrupt-cells" -> Seq(ResourceInt(1)))
       Description(name, mapping ++ extra)
     }