powerpc/perf/hv-24x7: Fix usage with chip events.

24x7 counters can belong to different domains (core, chip, virtual CPU
etc). For events in the 'chip' domain, sysfs entry currently looks like:

	$ cd /sys/bus/event_source/devices/hv_24x7/events
	$ cat PM_XLINK_CYCLES__PHYS_CHIP
	domain=0x1,offset=0x230,core=?,lpar=0x0

where the required parameter, 'core=?' is specified with perf as:

	perf stat -C 0 -e hv_24x7/PM_XLINK_CYCLES__PHYS_CHIP,core=1/ \
		/bin/true

This is inconsistent in that 'core' is a required parameter for a chip
event.  Instead, have the the sysfs entry display 'chip=?' for chip
events:

	$ cd /sys/bus/event_source/devices/hv_24x7/events
	$ cat PM_XLINK_CYCLES__PHYS_CHIP
	domain=0x1,offset=0x230,chip=?,lpar=0x0

We also need to add a 'chip' entry in the sysfs format directory:

	$ ls /sys/bus/event_source/devices/hv_24x7/format
	chip  core  domain  lpar  offset  vcpu
	^^^^
	(new)

so the perf tool can automatically check usage and format the chip
parameter correctly:

	$ perf stat -C 0 -v -e hv_24x7/PM_XLINK_CYCLES__PHYS_CHIP/ \
		/bin/true
	Required parameter 'chip' not specified
	invalid or unsupported event: 'hv_24x7/PM_XLINK_CYCLES__PHYS_CHIP/'

	$ perf stat -C 0 -v -e hv_24x7/PM_XLINK_CYCLES__PHYS_CHIP,chip=1/ \
		/bin/true
	hv_24x7/PM_XLINK_CYCLES__PHYS_CHIP,chip=1/: 0 6628908 6628908

	 Performance counter stats for 'CPU(s) 0':

	         0      hv_24x7/PM_XLINK_CYCLES__PHYS_CHIP,chip=1/

	    0.006606970 seconds time elapsed

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