Add field values to XML register description. (#727)

* Add field values to XML register description.

Generate constants in the C header for the values. E.g.
```
 #define CSR_TDATA1_TYPE_NONE                0
 #define CSR_TDATA1_TYPE_LEGACY              1
 #define CSR_TDATA1_TYPE_MCONTROL            2
 #define CSR_TDATA1_TYPE_ICOUNT              3
 ...
```
Make formatting slightly more consistent in the PDF.

* Use \unspecified instead of "undefined values"

* Rename a bunch of field values based on review.

* Rename field values that specify a number of bits

* stop{time,count} values are called normal/freeze

* Abstract command write value arg->arg0

* Name field values <n>bit