irqdomain: Make build work for CONFIG_GENERIC_IRQ_CHIP=n

ld: kernel/irq/irqdomain.o: in function `irq_domain_instantiate':
kernel/irq/irqdomain.c:296:(.text+0x10dd): undefined reference to `irq_domain_alloc_generic_chips'
ld: kernel/irq/irqdomain.c:313:(.text+0x1218): undefined reference to `irq_domain_remove_generic_chips'
ld: kernel/irq/irqdomain.o: in function `irq_domain_remove':
kernel/irq/irqdomain.c:349:(.text+0x1ddf): undefined reference to `irq_domain_remove_generic_chips'

Provide the required stubs.

Fixes: e6f67ce3 ("irqdomain: Add support for generic irq chips creation before publishing a domain")
Reported-by: Borislav Betkov <bp@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>