xen: pcpu: Use static attribute groups for sysfs entry

Instead of manual calls of device_create_file() and
device_remove_file(), assign the static attribute groups to the device
to register.  The conditional build of sysfs is done in is_visible
callback instead.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>