ARM DPM: share debug reason logic

ARM DPM: share debug reason logic

No point in both ARM11 and Cortex-A8 having private copies
of the logic sorting out e.g. DBG_REASON_WATCHPOINT.

Add and use a shared routine for this ... there's actually
a bunch more debug entry logic that could be shared, this
is just a start on that.  Note that this routine fixes a
bug observed in the ARM11 code, where some abort mode quirks
were displayed as being an unknown debug reason; and also
silences needless ARM11 chatter.

Likewise with private copies of DSCR ... add one to the DPM
struct.  Save it as part of setting DBG_REASON_* so later
patches can switch over to using that copy.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>