aarch64: handle exceptions taken in debug state

aarch64: handle exceptions taken in debug state

When an armv8-a PE causes an exception while halted, e.g. by performing
a prohibited memory or register access, its state is affected in the
same way as if it was running. That means, a number of registers is
overwritten (notably DLR and DSPSR, but also others) and also
potentially the exception level and therefore also the PE state can
change. This state must be restored before resuming normal operation.

This is done by marking the relevant cached registers "dirty" so that
they are written back before resume.

Change-Id: I9b6967a62d7cb23a477a9f7839f8d2b7087eed09
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>