target-arm: Implement AArch64 ID and feature registers

target-arm: Implement AArch64 ID and feature registers

Implement the AArch64-specific ID and feature registers. Although
many of these are currently not used by the architecture (and so
always zero for all implementations), we define the full set of
fields in the ARMCPU struct for symmetry.

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