hw/intc/arm_gic: Handle grouping for GICC_HPPIR

Grouping (GICv2) and Security Extensions change the behaviour of reads
of the highest priority pending interrupt register (ICCHPIR/GICC_HPPIR).

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: 1430502643-25909-12-git-send-email-peter.maydell@linaro.org
Message-id: 1429113742-8371-12-git-send-email-greg.bellows@linaro.org
[PMM: make utility fn static; coding style fixes; AckCtl has an effect
 for GICv2 without security extensions as well; removed checks on enable
 bits because these are done when we set current_pending[cpu]]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
