hw/intc/arm_gic: Restrict priority view

GICs with Security Extensions restrict the non-secure view of the
interrupt priority and priority mask registers.

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: 1430502643-25909-11-git-send-email-peter.maydell@linaro.org
Message-id: 1429113742-8371-15-git-send-email-greg.bellows@linaro.org
[PMM: minor code tweaks; fixed missing masking in gic_set_priority_mask
and gic_set_priority]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
