irqchip/irq-vt8500: Use a dedicated chained handler function

Current code for the chained interrupt controller maps its interrupts on
the parent but doesn't register a separate chained handler, instead
needlessly calling enable_irq() on an unactivated parent interrupt, causing
a boot time WARN_ON from the common code.

The common handler meanwhile loops through all registered interrupt
controllers in an arbitrary order and tries to handle active interrupts
in each of them, which is fragile.

Use common infrastructure for handling chained interrupts instead.

Signed-off-by: Alexey Charkov <alchark@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250506-vt8500-intc-updates-v2-4-a3a0606cf92d@gmail.com