powerpc/perf/hv-24x7: Display domain indices in sysfs

To help users determine domains, display the domain indices used by the
kernel in sysfs.

	$ cat /sys/bus/event_source/devices/hv_24x7/interface/domains
	1: Physical Chip
	2: Physical Core
	3: VCPU Home Core
	4: VCPU Home Chip
	5: VCPU Home Node
	6: VCPU Remote Node

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>