irqchip/mips-gic: Only register IPI domain when SMP is enabled

The MIPS GIC irqchip driver may be selected in a uniprocessor
configuration, but it unconditionally registers an IPI domain.

Limit the part of the driver dealing with IPIs to only be compiled when
GENERIC_IRQ_IPI is enabled, which corresponds to an SMP configuration.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220701200056.46555-2-samuel@sholland.org