powerpc/irq: Split irq.c

More than half of irq.c is dedicated to PPC64.

Move PPC64 code out of irq.c into irq_64.c

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9f1a47de80f78d3dd270a7a72f69f55f581c4054.1652859593.git.christophe.leroy@csgroup.eu