irqchip/loongson-pch-lpc: Extract non-ACPI-related code from ACPI init

irqchip/loongson-pch-lpc: Extract non-ACPI-related code from ACPI init

A lot of code can be shared between the existing ACPI init flow with the
upcoming OF init flow.

Extract it into a dedicated function.

The re-ordering of parent interrupt allocation requires the architecture
code to reserve legacy interrupts from the dynamic allocation by overriding
arch_dynirq_lower_bound(), otherwise the parent of LPC irqchip will be
allocated in the intended static range of LPC interrupts, which leads to
allocation failure of LPC interrupts.

Co-developed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Icenowy Zheng <zhengxingda@iscas.ac.cn>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://patch.msgid.link/20260321092032.3502701-5-zhengxingda@iscas.ac.cn