increase DTMCS.abits to be compliant debug specification

ROI from "The RISC-V Debug Specification" (1.0)

```
3.1. Debug Module Interface (DMI)
...
The DMI uses between 7 and 32 address bits. ...
```

NOTE: RISC-V External Debug Specification 0.13.2 has the same sentence.

riscv/jtag_dtm.h

@@ -51,7 +51,9 @@ class jtag_dtm_t
 
     // abits must come before dtmcontrol so it can easily be used in the
     // constructor.
-    const unsigned abits = 6;
+    // From RISC-V Debug Spec (both 0.13.2 and 1.0):
+    // The DMI uses between 7 and 32 address bits.
+    const unsigned abits = 7;
     uint32_t dtmcontrol;
     uint64_t dmi;
     unsigned bypass;