irqchip/loongson-eiointc: Route interrupt parsed from bios table

Interrupt controller eiointc routes interrupts to CPU interface IP0 - IP7.

It is currently hard-coded that eiointc routes interrupts to the CPU
starting from IP1, but it should base that decision on the parent
interrupt, which is provided by ACPI or DTS.

Retrieve the parent's hardware interrupt number and store it in the
descriptor of the eointc instance, so that the routing function can utilize
it for the correct route settings.

[ tglx: Massaged change log ]

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250804081946.1456573-2-maobibo@loongson.cn