hw/arm_gic: Correctly restore nested irq priority

hw/arm_gic: Correctly restore nested irq priority

Upon activating an interrupt, set the corresponding priority bit in the
APR/NSAPR registers without touching the currently set bits. In the event
of nested interrupts, the GIC will then have the information it needs to
restore the priority of the pre-empted interrupt once the higher priority
interrupt finishes execution.

Signed-off-by: François Baldassari <francois@pebble.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>