Clean up treatment of registers in ARMv7-M and Cortex-M3. 

 - At the arch level:
    * Just list registers and names; don't impose core-specific
      policy about how they are accessed.
    * Each register has a symbol.
    * Remove the register mode field (irrelevant to debugger)

 - At the core/implementation level:
    * Just map the registers to their relevant access methods;
      don't require the arch level to say how that should work
      (cores other than Cortex-M3 could do it differently).
    * Don't use undefined bits from register 20.
    * Use register IDs that are part of the ARMv7-M interface.

In short, there's now a real distinction between the arch
and core layers.

git-svn-id: svn://svn.berlios.de/openocd/trunk@2554 b42882b7-edfa-0310-969c-e2dbd0fdcd60