irqdomain: Clean up irq_domain_push/pop_irq()

The irq_domain_push_irq() interface is used to add a new (outmost) level
to a hierarchical domain after IRQs have been allocated.

Possibly due to differing mental images of hierarchical domains, the
names used for the irq_data variables make these functions much harder
to understand than what they need to be.

Rename the struct irq_data pointer to the data embedded in the
descriptor as simply 'irq_data' and refer to the data allocated by this
interface as 'parent_irq_data' so that the names reflect how
hierarchical domains are implemented.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
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/20230213104302.17307-12-johan+linaro@kernel.org