hw/intc/arm_gic: Implement Non-secure view of RPR

For GICs with Security Extensions Non-secure reads have a restricted
view on the current running priority.

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: 1430502643-25909-10-git-send-email-peter.maydell@linaro.org
Message-id: 1429113742-8371-11-git-send-email-greg.bellows@linaro.org
[PMM: make function static, minor comment tweak]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
