powerpc/perf/hv-24x7: Move cpumask file to top folder of hv-24x7 driver

Commit 792f73f747 ("powerpc/hv-24x7: Add sysfs files inside hv-24x7
device to show cpumask") added cpumask file as part of hv-24x7 driver
inside the interface folder. The cpumask file is supposed to be in the
top folder of the PMU driver in order to make hotplug work.

This patch fixes that issue and creates new group 'cpumask_attr_group'
to add cpumask file and make sure it added in top folder.

  command:# cat /sys/devices/hv_24x7/cpumask
  0

Fixes: 792f73f747 ("powerpc/hv-24x7: Add sysfs files inside hv-24x7 device to show cpumask")
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200821080610.123997-1-kjain@linux.ibm.com

Documentation/ABI/testing/sysfs-bus-event_source-devices-hv_24x7

@@ -43,7 +43,7 @@ Description:	read only
 		This sysfs interface exposes the number of cores per chip
 		present in the system.
 
-What:		/sys/devices/hv_24x7/interface/cpumask
+What:		/sys/devices/hv_24x7/cpumask
 Date:		July 2020
 Contact:	Linux on PowerPC Developer List <linuxppc-dev@lists.ozlabs.org>
 Description:	read only