topology/sysfs: Hide PPIN on systems that do not support it.

Systems that do not support a Protected Processor Identification Number
currently report:

	# cat /sys/devices/system/cpu/cpu0/topology/ppin
	0x0

which is confusing/wrong.

Add a ".is_visible" function to suppress inclusion of the ppin file.

Fixes: ab28e944

 ("topology/sysfs: Add PPIN in sysfs under cpu topology")
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20220406220150.63855-1-tony.luck@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>