target-arm: Implement missing AFSR registers

The AFSR registers are implementation dependent auxiliary fault
status registers. We already implemented a RAZ/WI AFSR0_EL1 and
AFSR_EL1; add the missing AFSR{0,1}_EL{2,3} for consistency.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1438281398-18746-4-git-send-email-peter.maydell@linaro.org