Add ASID field to SATP

ASID field is missing from the SATP which causes compatibility
issues with Xous.

While this patch resolves the Xous issue, it has not been tested
on Linux.

src/main/scala/vexriscv/plugin/MmuPlugin.scala

@@ -91,11 +91,12 @@ class MmuPlugin(ioRange : UInt => Bool,
       }
       val satp = new Area {
         val mode = RegInit(False)
+        val asid = Reg(Bits(9 bits))
         val ppn = Reg(UInt(20 bits))
       }
 
       for(offset <- List(CSR.MSTATUS, CSR.SSTATUS)) csrService.rw(offset, 19 -> status.mxr, 18 -> status.sum, 17 -> status.mprv)
-      csrService.rw(CSR.SATP, 31 -> satp.mode, 0 -> satp.ppn)
+      csrService.rw(CSR.SATP, 31 -> satp.mode, 22 -> satp.asid, 0 -> satp.ppn)
     }
 
     val core = pipeline plug new Area {