hw/intc/arm_gic: Change behavior of EOIR writes

Grouping (GICv2) and Security Extensions change the behavior of EOIR
writes. Completing Group0 interrupts is only allowed from Secure state.

Signed-off-by: Fabian Aggeler <aggelerf@ethz.ch>
Signed-off-by: Greg Bellows <greg.bellows@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1430502643-25909-13-git-send-email-peter.maydell@linaro.org
Message-id: 1429113742-8371-13-git-send-email-greg.bellows@linaro.org
[PMM: Rather than go to great lengths to ignore the UNPREDICTABLE case
 of a Secure EOI of a Group1 (NS) irq with AckCtl == 0, we just let
 it fall through; add a comment about it.]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
