irqchip/irq-vt8500: Use fewer global variables and add error handling

Controller private data doesn't really need to be in a global
statically allocated array - kzalloc it per controller instead,
keeping only one pointer to the primary controller global.

While at that, also add proper error return statuses in the init
path and respective cleanup of resources on errors.

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-v2-5-a3a0606cf92d@gmail.com