Fix issue 90

[tariqkurd-repo]

Changes are:

1. pcc is no longer accessible as a CSR
2. the pcc value is UNSPECIFIED in debug mode, and ASR permission is implicitly granted
3. add dinfc CSR as a debug mode only source of infinity
4. remove ASR permission from all debug mode CSRs as it's implicitly granted
5. remove "M-mode" as a prerequisite for (e.g.) MTVECC as M-mode must always be present

[arichardson]

Not sure about the dinfc R/W designation otherwise LGTM.

[sorear]

I still think that a Rocket-style debug ROM implementation will require D mode to always execute using the capability ISA, but that can be discussed as a separate issue.

We're no longer violating the debug spec's own rules on PC access, so approve.

[andresag01]

Just a small formatting change. Otherwise looks good to me.