irqchip/gic: Don't unnecessarily write the IRQ configuration

If the interrupt configuration matches the current configuration, then
don't bother writing the configuration again.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>