target-arm: Implement auxiliary fault status registers

target-arm: Implement auxiliary fault status registers

Implement the auxiliary fault status registers AFSR0_EL1 and
AFSR1_EL1. These are present on v7 and later, and have IMPDEF
behaviour; we choose to RAZ/WI for all cores.

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