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

arch/powerpc/kernel/Makefile

@@ -63,7 +63,7 @@ obj-y				:= cputable.o syscalls.o \
 				   hw_breakpoint_constraints.o interrupt.o \
 				   kdebugfs.o stacktrace.o
 obj-y				+= ptrace/
-obj-$(CONFIG_PPC64)		+= setup_64.o \
+obj-$(CONFIG_PPC64)		+= setup_64.o irq_64.o\
 				   paca.o nvram_64.o note.o
 obj-$(CONFIG_COMPAT)		+= sys_ppc32.o signal_32.o
 obj-$(CONFIG_VDSO32)		+= vdso32_wrapper.o