irqchip/gic-v3-its: Use irq_domain_create_hierarchy()

irqchip/gic-v3-its: Use irq_domain_create_hierarchy()

Use the irq_domain_create_hierarchy() helper to create the hierarchical
domain, which both serves as documentation and avoids poking at
irqdomain internals.

Note that the domain host_data was first set to the struct its_node
during allocation only to immediately be overwritten with the struct
msi_domain_info.

Tested-by: Hsin-Yi Wang <hsinyi@chromium.org>
Tested-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230213104302.17307-17-johan+linaro@kernel.org