genirq: Deduplicate interrupt descriptor initialization

alloc_desc() and early_irq_init() contain duplicated code to initialize
interrupt descriptors.

Replace that with a helper function.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Dawei Li <dawei.li@shingroup.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240122085716.2999875-6-dawei.li@shingroup.cn