irqchip/mips-cpu: Set IPI domain parent chip

Since commit 55567976 ("genirq/irqdomain: Allow partial trimming of
irq_data hierarchy") the irq_data chain is valided.

The irq_domain_trim_hierarchy() function doesn't consider the irq + ipi
domain hierarchy as valid, since the ipi domain has the irq domain set
as parent, but the parent domain has no chip set. Hence the boot ends in
a kernel panic.

Set the chip for the parent domain as it is done in the mips gic irq
driver, to have a valid irq_data chain.

Fixes: 3838a547

 ("irqchip: mips-cpu: Introduce IPI IRQ domain support")
Cc: <stable@vger.kernel.org> # v5.10+
Signed-off-by: Mathias Kresin <dev@kresin.me>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210107213603.1637781-1-dev@kresin.me