irqchip/gic-v3: Report failures in gic_irq_domain_alloc

If the GIC cannot map an IRQ via irq_domain_ops->alloc(), it doesn't
return an error code.  This can cause a problem with drivers, where
it thinks it has successfully got an IRQ for the device, but requesting
the same ends up failure with -ENOSYS (as the IRQ's chip is not set).

Fixes: commit 443acc4f ("irqchip: GICv3: Convert to domain hierarchy")
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>