target-arm: Implement AArch64 view of CPACR

target-arm: Implement AArch64 view of CPACR

Implement the AArch64 view of the CPACR. The AArch64
CPACR is defined to have a lot of RES0 bits, but since
the architecture defines that RES0 bits may be implemented
as reads-as-written and we know that a v8 CPU will have
no registered coprocessors for cp0..cp13 we can safely
implement the whole register this way.

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