Add Book-E Enhanced Debug (E.D) profile debug support

Freescale added the E.D profile to e500mc and derivative cores. From
Freescale's EREF reference manual this is enabled by a bit in HID0 and should
otherwise default to traditional debug. However, none of the Freescale cores
support that bit, and instead always use E.D. This results in kernel panics
using the standard debug on e500mc+ cores.

Enhanced debug allows debugging of interrupts, including critical interrupts,
as it uses a different save/restore registers (srr*). At this time we don't use
this ability, so instead share the core of the debug handler code between both
handlers.

MFC after: 3 weeks