MIPS: Kill RM7K & RM9K IRQ Code

RM7000 IRQ driver never got really used by any of the platform,
and rm9k_cpu_irq_init only exist in a header.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>