target-arm: Implement Cortex-A57 implementation-defined system registers

target-arm: Implement Cortex-A57 implementation-defined system registers

Implement a subset of the Cortex-A57's implementation defined system
registers. We provide RAZ/WI or reads-as-constant/writes-ignored
implementations of the various control and syndrome reigsters.
We do not implement registers which provide direct access to and
manipulation of the L1 cache, since QEMU doesn't implement caches.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>