target-arm: Collect up the debug cp register definitions

At the moment we have a mixed set of mostly dummy register
definitions for various debug related registers which have
been added piecemeal in order to get Linux kernels to boot.
In preparation for actually implementing debug support,
bring them all together into one place.

This commit doesn't change behaviour: we still expose
exactly the same registers and behaviour to the guest
in all configurations.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
